Vertical color filter sensor group with carrier-collection elements of different size and method for fabricating such a sensor group

ABSTRACT

A vertical color filter sensor group formed on a substrate (preferably a semiconductor substrate) and including at least two vertically stacked, photosensitive sensors, and an array of such sensor groups. In some embodiments, a carrier-collection element of at least one sensor of the group has substantially larger area, projected in a plane perpendicular to a normal axis defined by a top surface of a top sensor of the group, than does each minimum-sized carrier-collection element of the group. In some embodiments, the array includes at least two sensor groups that share at least one carrier-collection element. Optionally, the sensor group includes at least one filter positioned relative to the sensors such that radiation that has propagated through or reflected from the filter will propagate into at least one sensor of the group.

RELATED APPLICATIONS

This application is a continuation in part of U.S. patent applicationSer. No. 10/103,304, entitled VERTICAL COLOR FILTER DETECTOR GROUP ANDARRAY, filed on Mar. 20, 2002.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to photosensitive sensor groups thatcomprise vertically stacked sensors. In each group, semiconductormaterial chromatically filters incident electromagnetic radiationvertically (optionally, other material also filters the radiation) andeach sensor simultaneously detects a different wavelength band. Theinvention also relates to arrays of such sensor groups, with each sensorgroup positioned at a different pixel location.

2. Background of the Invention

The expressions “filter” and “color filter” are used interchangeablyherein (including in the claims) in a broad sense to denote an elementthat selectively transmits or reflects at least one wavelength band ofelectromagnetic radiation that is incident thereon. For example, onetype of filter is a dichroic mirror that both transmits radiation in afirst wavelength band and reflects radiation in a second wavelengthband. Examples of filters include short wave pass filters, long wavepass filters, and band pass filters.

The term “radiation” is used herein to denote electromagnetic radiation.

The expression “top sensor” (of a sensor group) herein denotes thesensor of the group that radiation, incident at the sensor group,reaches before reaching any other sensor of the group. The expressionthat the sensors of a sensor group are “vertically stacked” denotes thatone of the sensors is a top sensor of the group, and that the group hasan axis (sometimes referred to as a “vertical axis”) that extendsthrough all the sensors. As described below, a vertical color filter(“VCF”) sensor group that embodies the invention preferably includesvertically stacked sensors configured such that the group's top sensorhas a top surface that defines a normal axis (e.g., is at leastsubstantially planar), and when radiation propagating along a verticalaxis of the group is incident at the group, the radiation is incident atthe top sensor with an incidence angle of less than about 30 degreeswith respect to the normal axis (e.g., the radiation is normallyincident at the group).

The expression used herein that two elements, included in a structurehaving a vertical axis, are “laterally” (or “horizontally”) separateddenotes that there is an axis parallel to the vertical axis that extendsbetween the elements but intersects neither element.

The expression that an item “comprises” an element is used herein(including in the claims) to denote that the item is or includes theelement.

MOS active pixel sensors are known in the art. Multiple-wavelength bandactive pixel sensor arrays are also known in the art. One type ofmultiple-wavelength band active pixel sensor array employs red, green,and blue sensors disposed horizontally in a pattern at or near thesemiconductor surface. Color overlay filters are employed to produce thecolor selectivity between the red, green, and blue sensors. Such sensorshave the disadvantage of occupying a relatively large area perresolution element as these sensors are tiled together in a plane. Inaddition, reconstruction of a color image from such a sensor array iscomputationally intensive and often results in images with artifacts,defects, or inferior resolution.

Another type of multiple-wavelength band pixel sensor array employsgroups of sensors, each group including sensors in a vertically-orientedarrangement. An example of an early multiple-wavelength vertical sensorgroup for detecting visible and infra-red radiation is disclosed in U.S.Pat. No. 4,238,760 to Carr, in which a first diode in a surface n-typeepitaxial region is responsive to visible light and a second diode(including a buried p-region in an underlying n-type substrate) isresponsive to infrared radiation. Carr teaches that contact to theburied diode is made using a deep diffusion process “similar todiffusion-under-film collector contact diffusion common in bipolar ICprocessing and for reducing the parameter R_(CS).” Carr also disclosesan embodiment in which a V-groove contact (created by a process thatincludes a step of etching through the n-type epitaxial region) providescontact to the buried p-type region. The disclosed device has a size of4 mils square. The device disclosed in the Carr patent has severalshortcomings, the most notable being its large area, rendering itunsuitable for the image sensor density requirements of modern imagingsystems. The technology employed for contact formation to the buriedinfrared sensing diode is not suitable for modern imaging technology orextension to a 3-color sensor.

U.S. Pat. No. 5,965,875 to Merrill discloses a three-color, visiblelight, sensor group in which a structure is provided using a triple-wellCMOS process wherein the blue, green, and red sensitive PN junctions aredisposed at different depths relative to the surface of thesemiconductor substrate upon which the imager is fabricated. Thisthree-color sensor group permits fabrication of a dense imaging arraybecause the three colors are sensed over approximately the same area inthe image plane. However, its structure has several shortcomings. First,the sensor group uses a reverse-polarity central green-sensitive PNjunction, requiring modified circuits or voltage ranges, possiblyinvolving PMOS transistors in addition to the usual NMOS transistors, tosense and read out the green channel. This requirement disadvantageouslyincreases sensor area and complicates support circuits in detectors thatinclude the sensor groups. The added circuit complexity makes itdifficult to make an image sensor array that has flexible color readoutcapabilities (as disclosed herein) and makes it impossible to achievethe small sensor size required by many modern electronic imagingapplications.

U.S. Pat. No. 6,111,300 to Cao, et al., discloses a color active pixelsensor which uses a PIN photodiode to attempt to collect blue light, andtwo additional semiconductor junction diodes (vertically spaced from thePIN photodiode) within a semiconductor substrate to detect green and redlight. Among the shortcomings of this sensor are difficult andnon-standard manufacturing techniques, use of structures that prohibithigh density of sensors (in an array), no ability to select differentcolors for read out, and inability to perform detection of three or morecolors using a monolithic semiconductor substrate.

Findlater et al. (“A CMOS Image Sensor Employing a Double JunctionPhotodiode,” K. M. Findlater, D. Renshaw, J. E. D. Hurwitz, R. K.Henderson, T. E. R. Bailey, S. G. Smith, M. D. Purcell, and J. M.Raynor, in 2001 IEEE Workshop on Charge-Coupled Devices and AdvancedImage Sensors, IEEE Electron Devices Society (2001)) disclose an activepixel sensor that employs a double-junction photodiode in conjunctionwith an organic filter overlay. Each double-junction photodiodecomprises top and bottom p-type layers with an n-type layer betweenthem. The n-type layer forms the cathode of a first photodiode, thebottom p-type layer forms the anode of a second photodiode, the firstphotodiode is coupled to a first readout circuit, and the secondphotodiode is coupled to a second readout circuit. A mosaic of cyan andyellow filters overlays an array of the sensors so that in each row ofthe array, the even-numbered sensors receive a radiation in a firstwavelength band (blue and green) and the odd-numbered sensors receiveradiation in a second wavelength band (red and green). The performanceof an array of such sensors is limited by the poor color response of thedouble-junction photodiode and by the fact that the n-well forms thecathode of both photodiodes, making the sensor design very susceptibleto non-linear crosstalk between the color channels. Additionally, theauthors cite non-uniformity and process/fabrication constraints thatlimit the performance and potential benefits of this design.

Several types of vertical color filter (“VCF”) sensor groups and methodsfor fabricating them are described in above-referenced U.S. patentapplication Ser. No. 09/884,863, and in above-referenced U.S. patentapplication Ser. No. 10/103,304. A VCF sensor group includes at leasttwo photosensitive sensors that are vertically stacked with respect toeach other (with or without non-sensor material between adjacentsensors). Each sensor of a VCF sensor group has a different spectralresponse. Typically, each sensor has a spectral response that peaks at adifferent wavelength. In some embodiments, a VCF sensor group (or one ormore of the sensors thereof) includes a filter that does not alsofunction as a sensor.

A VCF sensor group simultaneously senses photons of at least twowavelength bands in the same area of the imaging plane. In contrast,time sequential photon sensing methods do not perform photon sensing atthe same time for all wavelength bands. The sensing performed by a VCFsensor group included in an imager occurs in one area of the imager(when the imager is viewed vertically), and photons are separated bywavelength as a function of depth into the sensor group.

Typically, each sensor detects photons in a different wavelength band(e.g., one sensor detects more photons in the “blue” wavelength bandthan each other sensor, a second sensor detects more photons in the“green” wavelength band than each other sensor, and a third sensordetects more photons in the “red” wavelength band than each othersensor), although the sensor group typically has some “cross-talk” inthe sense that multiple sensors detect photons of the same wavelength.

VCF sensor groups can be used for a variety of imaging tasks. Inpreferred embodiments, they are used in digital still cameras (DSC).However they can be employed in many other systems, such as linearimagers, video cameras and machine vision equipment.

A VCF sensor group uses the properties of at least one semiconductormaterial to detect incident photons, and also to selectively detectincident photons of different wavelengths at different depths in thegroup. The detection of different wavelengths is possible due to thevertical stacking of the sensor layers of the sensor group incombination with the variation of optical absorption depth withwavelength in semiconductor materials. The costs of manufacturing VCFsensor groups are substantially reduced because VCF sensor groups do notrequire external color filters (as are traditionally used in color imagesensors) and do not require color filters that are distinct from thesensors themselves (the sensors themselves are made of semiconductormaterial that itself provides a filtering function). However, in someembodiments of the invention, VCF sensor groups do include (or are usedwith) color filters that are distinct from the sensors themselves. Thespectral response characteristics of VCF color sensor groups typicallyare much more stable and less sensitive to external factors such astemperature or other environmental factors (that may be present duringor after manufacturing) than are conventional color sensors withnon-semiconductor based filters.

A VCF sensor group is preferably formed on a substrate (preferably asemiconductor substrate) and comprises a plurality of vertically stackedsensors (e.g., sensor layers) configured by doping and/or biasing tocollect photo-generated carriers of a first polarity (preferablynegative electrons). The sensors include (or pairs of the sensors areseparated by) one or more reference layers configured to collect andconduct away photo-generated carriers of the opposite polarity(preferably positive holes). The sensors have different spectralsensitivities based on their different depths in the sensor group, andon other parameters including doping levels and biasing conditions. Inoperation, the sensors are individually connected to biasing and activepixel sensor readout circuitry. VCF sensor groups and methods forfabricating them are discussed more fully in above-referenced U.S.patent application Ser. No. 09/884,863, and in the parent application,U.S. patent application Ser. No. 10/103,304.

An array of VCF sensor groups can be modified by positioning a patternof color filters over the array, as described in U.S. patent applicationSer. No. 10/103,304. Using filters made of only a single filter materialand positioned over a subset of the sensor groups, an array with threesensors per sensor group can be operated to detect radiation in four,five, or six different wavelength bands (by reading out signals fromdifferent selected subsets of the sensor groups of the array). This canyield improved color accuracy. Any of many different types of filterscan be employed, including organic dye filters as in some conventionalcolor image sensors, and filters comprising one or more layers that areintegrated with the sensor group by a semiconductor integrated circuitfabrication process (e.g., a layer of polysilicon to absorb shortwavelengths, an interference filter that is a stack of alternating oxideand nitride layers, or another interference filter for shaping thespectral response by interference effects).

BRIEF DESCRIPTION OF THE INVENTION

In a class of embodiments, the invention is a vertical color filter(VCF) sensor group formed on a substrate (preferably a semiconductorsubstrate) and including at least two vertically stacked, photosensitivesensors. In some preferred embodiments of the inventive VCF sensorgroup, the carrier-collection element of one sensor of the group hassubstantially larger “size” (projected area in a plane perpendicular toa normal axis defined by the top surface of a top sensor of the group)than does each minimum-sized carrier-collection element of the group,where “minimum-sized” carrier-collection element denotes eachcarrier-collection element of the group whose projection on such planehas an area that is less than or equal to the projected area on suchplane of each other carrier-collection element of the group. Inpreferred embodiments in this class, one carrier-collection element ofthe group has size that is at least twice the group's minimum collectionarea, where “minimum collection area” denotes the size of aminimum-sized carrier-collection element of the group. In another classof embodiments, the invention is an array of VCF sensor groups, whereineach of the sensors of each of the sensor groups has acarrier-collection element, and at least two of the sensor groups“share” at least one carrier-collection element, in the sense that avertical axis of each sensor group that “shares” a carrier-collectionelement intersects such carrier-collection element. When readoutcircuitry is coupled to the sensor groups of an array in this class, aphotogenerated carrier signal read from a shared carrier-collectionelement can be used to generate an output signal of each sensor groupthat shares the carrier-collection element. In a preferred embodiment ofan array in this class, each sensor group includes a blue sensor, agreen sensor, and a red sensor, the carrier-collection elements of thered and blue sensors of each group have larger size than does thecarrier-collection element of the group's green sensor, and thecarrier-collection element of the group's red or blue sensor is (or thecarrier-collection elements of both the red and blue sensors are) sharedwith at least one other sensor group. Preferably, the carrier-collectionelement of each group's green sensor is not shared with any other sensorgroup of the array.

Some embodiments of the inventive array are one-dimensional sensorarrays; others are two-dimensional sensor arrays.

Optionally, the inventive sensor group (or each of one or more sensorgroups of an array that embodies the invention) includes at least onefilter positioned relative to the sensors of the group such thatradiation that has propagated through or reflected from the filter willpropagate into at least one sensor of the group.

Another aspect of the invention is an image detector that comprises atleast one array of VCF sensor groups and circuitry for convertingphotogenerated carriers produced in the sensors to electrical signals.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of the intensity of electromagnetic radiation incrystalline silicon (relative to its incident intensity I₀) as afunction of depth (in microns) in the silicon, for the wavelengths 450nm, 550 nm, and 650 nm.

FIG. 2 is a graph indicative of a vertical doping profile for a VCFsensor group that embodies the invention.

FIG. 2A is a cross-sectional view (in a vertical plane) of the VCFsensor group whose profile is shown in FIG. 2, with a schematic circuitdiagram of biasing and readout circuitry coupled to the sensor group.

FIG. 3 is a graph of the absorption rate of electromagnetic radiation incrystalline silicon (relative to its incident intensity I₀) as afunction of depth (in microns) in the silicon, for the wavelengths 450nm (curve A), 550 nm (curve B), and 650 nm (curve C), with indicationsof the locations of the FIG. 2 sensor group's layers overlayed thereon.

FIG. 4 is a graph of the spectral response of the three photodiodes ofthe sensor group whose profile is similar to that shown in FIG. 2.

FIG. 5 is a simplified cross-sectional view (in a vertical plane) of anembodiment of the inventive VCF sensor group.

FIG. 6 is a table that lists (in the center column) the bandgap energyin electron volts of In_(x)Ga_(1-x)N semiconductor having differentlevels of Indium content, and (in the right column) the opticalwavelength corresponding to each bandgap energy.

FIG. 7 is a cross-sectional view of an avalanche sensor that can beincluded in an embodiment of the inventive VCF sensor group.

FIG. 8 is a cross-section view of a portion of an array of the inventiveVCF sensor groups, each sensor group in the array including twonon-sensor filters and three sensors.

FIG. 8A is a simplified top view of a portion of an array of theinventive VCF sensor groups, in which each of the groups that includes afilter is marked with an “X.”

FIG. 8B is a simplified top view of a portion of another array of theinventive VCF sensor groups, in which each of the groups that includes afilter is marked with an “X.”

FIG. 9 is a cross-section view of a portion of an array of the inventiveVCF sensor groups, in which a micro-lens is formed over each sensorgroup of the array.

FIG. 10 is a simplified top view of a portion of an array of theinventive VCF sensor groups, in which adjacent sensor groups sharecarrier-collection elements.

FIG. 10A is a cross-sectional view (in a vertical plane) of two VCFsensor groups of an array, in which two sensor groups share a commonsensor element.

FIG. 10B is a top view of four VCF sensor groups of an array, in whichthe four sensor groups share carrier-collection areas for collectingcarriers that have been photo-generated by absorption of red and bluephotons.

FIG. 11 is a cross-sectional view (in a vertical plane) of a portion ofa conventional sensor array.

FIG. 12 is a cross-sectional view (in a vertical plane) of a portion ofan array of VCF sensor groups, with trench isolation structures betweenadjacent sensor groups of the array.

FIGS. 13 a-13 f are cross-sectional views (in a vertical plane) ofstructures formed at various steps of manufacture of an embodiment ofthe inventive VCF sensor group.

FIGS. 14A-14L are cross-sectional views (in a vertical plane) ofstructures formed at various steps of manufacture of another embodimentof the inventive VCF sensor group.

FIGS. 15A-15H are cross-sectional views (in a vertical plane) ofstructures formed at various steps of manufacture of another embodimentof the inventive VCF sensor group.

FIGS. 16A-16H are cross-sectional views (in a vertical plane) ofstructures formed at various steps of manufacture of another embodimentof the inventive VCF sensor group.

FIG. 17 is a cross-sectional view (in a vertical plane) of a structureformed during manufacture of an embodiment of a VCF sensor group,including a plug contact formed by an implantation process. Each contour(representing the boundary between p-type and n-type material) indicatesthe result of forming the plug contact with a different-type dopinglevel, with the smallest n-type region having a first (“1×”) n-typedoping level, the largest n-type region having twice (“2×”) this dopinglevel, and the intermediate size n-type region having an intermediate(“1.4×”) n-type doping level.

FIG. 18 is a cross-sectional view (in a vertical plane) of a structureformed during manufacture of a preferred embodiment of the inventive VCFsensor group, including a bottom portion of a plug contact (formedduring an early stage of a multi-stage implantation process).

FIG. 18A is a cross-sectional view (in a vertical plane) of a structure,formed from the FIG. 18 structure during manufacture of a preferredembodiment of the inventive VCF sensor group, including a top portion ofthe plug contact (formed during a subsequent stage of the multi-stageimplantation process) whose bottom portion is shown in both FIGS. 18 and18A.

FIG. 19 is a graph of the mask thickness required during typicalimplantation of Boron, Phosphorus, Arsenic, and Antimony, for each offive indicated masking materials.

FIG. 20 is a simplified cross-sectional view (in a vertical plane) of anembodiment of the inventive VCF sensor group including a blanket barrierlayer (205) between two sensors.

FIG. 21 is a graph of dopant concentration as a function of depth in thesensor group of FIG. 20.

FIG. 22 is a simplified cross-sectional view (in a vertical plane) of avariation on the sensor group of FIG. 20, including conventional blanketbarrier implants rather than the inventive blanket barrier layer 205.

FIG. 23 is a graph of dopant concentration as a function of depth in thesensor group of FIG. 22.

FIG. 24 is a simplified cross-sectional view (in a vertical plane) ofanother embodiment of the inventive VCF sensor group, including ablanket barrier layer (205) between two sensors and additional blanketbarrier implants (207 and 208).

FIGS. 25A-25D are cross-sectional views (in a vertical plane) ofstructures formed at various steps of a self-aligned complementaryimplant process during manufacture of an embodiment of the inventive VCFsensor group.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Persons of ordinary skill in the art will realize that the followingdescription of the present invention is illustrative only and not in anyway limiting. Other embodiments of the invention will readily suggestthemselves to such skilled persons having the benefit of thisdisclosure.

Most of the fabrication processes to be described herein assume sensorsmade of crystal silicon, but the methods (or modifications thereof thatwill be apparent to those of ordinary skill in the art) typically alsoapply to sensors made of other semiconductor material or materials. Eachsensor of a VCF sensor group senses photons by directly or indirectlyconverting their energy into electron-hole pairs. This occurs insemiconducting material. A VCF sensor group is typically implemented sothat the output of each sensor in the group is indicative of a differentwavelength band of incident radiation. The radiation that reaches eachsensor in a VCF sensor group has a different wavelength-intensityspectrum due to the filtering action of the material forming the sensorgroup. Thus, all sensors in a VCF sensor group can be identical and eachsensor can still produce an output that is indicative of a differentwavelength band. In some embodiments, however, the sensors in a VCFsensor group are not all identical (e.g., they do not all consist of thesame material or combination of materials), and the structure andcomposition of each is determined so as to optimize or improve thesensor group's performance for a predetermined application. For example,a sensor having relatively high sensitivity to a given range ofwavelengths (i.e., relatively high absorptivity in such range) and lowersensitivity to other wavelengths, can be vertically stacked with sensorsmade of other materials having different spectral sensitivity to form aVCF sensor group.

Color output for a digital still camera (DSC) requires sensing of aminimum of three spectral bands due to the tri-chromatic nature of thehuman visual system. Thus, many embodiments of the inventive VCF sensorgroup have three vertically stacked sensors (each comprisingsemiconductor material) for sensing three different spectral bands. VCFsensor groups with two rather than three vertically stacked sensors areuseful in other applications, such as for simultaneous detection ofvisible and infrared radiation as described, for example, in U.S. Pat.No. 4,581,625 and U.S. Pat. No. 4,677,289. Since there can be advantagesto sensing more than three spectral regions, some embodiments of theinventive VCF sensor group have more than three vertically stackedsensors. Using the extra information from additional spectral regions,it can be possible to produce a more accurate representation of thecolor of an object. As more spectral data are available, the accuracy ofcolor representation potentially improves.

In a class of embodiments of the inventive VCF sensor group, each sensorincludes two layers of semiconductor material (as does the sensorcomprising layer X01 and an adjacent portion of layer X09 in FIG. 2) orthree layers of semiconductor material (as does the sensor comprisinglayer X02 and adjacent portions of layers X09 and X10 in FIG. 2), thereis a junction (e.g., a “p-n” junction or heterojunction) between eachtwo adjacent layers of a sensor, and one of the sensor's layers is acarrier-collection element having a contact portion (accessible tobiasing and readout circuitry). During typical operation, the layers ofeach sensor are biased so that photogenerated carriers migrate throughat least one depletion region to the contact to make a photochargesignal available at the contact portion. In typical embodiments of a VCFsensor group, the group includes material (e.g., the semiconductormaterial of layer X09 in FIG. 2 that belongs neither to depletion regionX04 nor depletion region X05) in which photons can be absorbed and suchabsorption is likely to produce charge that is detected by readoutcircuitry, but in which photogenerated carriers can migrate (withsignificant probability) toward any of at least two differentcarrier-collection elements. Typically, but not necessarily, all layersof a VCF sensor group consist of semiconductor material.

FIG. 1 is a graph of the intensity of electromagnetic radiation incrystalline silicon (relative to its incident intensity I₀) as afunction of depth in the silicon, for the wavelengths 450 nm, 550 nm,and 650 nm. FIG. 3 is a graph of the absorption rate of electromagneticradiation in crystalline silicon (relative to its incident intensity I₀)as a function of depth in the silicon, for the wavelengths 450 nm (curveA), 550 nm (curve B), and 650 nm (Curve C), with indications of thelocations of the FIG. 2 sensor group's layers overlayed thereon. Thegraphs of FIGS. 1 and 3 are generated from the same data. Each curve ofFIG. 3 plots difference values, with the “n”th difference value beingthe difference between the “(n+1)th” and “n”th data values of thecorresponding curve of FIG. 1. The intensity of radiation (having agiven wavelength) as a function of depth in many semiconductors otherthan silicon is a function similar to those graphed in FIG. 1. FIG. 1shows that (for each wavelength) the radiation's relative intensity (theratio I/I₀, where “I” is the intensity at depth “x” in the silicon and“I₀” is the incident intensity) decreases with increasing depth as thephotons are absorbed by the silicon. FIGS. 1 and 3 show that relativelymore blue (450 nm) photons are absorbed near the surface than arephotons of longer wavelength, and that at any depth in the silicon, moregreen (550 nm) photons than blue photons are present and that more red(650 nm) photons than green photons are present (assuming equal incidentintensity for red, green, and blue photons).

Each of the three curves of FIG. 1 (and FIG. 3) indicates an exponentialintensity drop off with increasing depth, and is based on the measuredbehavior of light in crystalline silicon that has been subjected totypical doping and processing. The exact shape of each curve will dependon the parameters of doping and processing, but there will be only smalldifferences between curves that assume different sets of doping and/orprocessing parameters. It is well known that the absorption of photonsof different wavelengths by a semiconductor depends on the bandgapenergy of the semiconductor material and on the details of the states atthe band edges. It is also well known that typical semiconductors (e.g.,silicon) have different absorptivity to different wavelengths.

As is apparent from FIGS. 1 and 3, a volume of silicon that functions asa sensor in a VCF sensor group at a given depth in a larger volume ofthe silicon, and has a given thickness, has greater absorptivity to bluelight than green light and greater absorptivity to green light than redlight. However, if the sensor silicon is sufficiently deep in the largervolume, most of the blue and green light will have been absorbed by thematerial above the sensor silicon. Even if light having a substantiallyflat wavelength-intensity spectrum is incident at the surface of thelarger volume, the sensor can actually absorb more red light than greenor blue light if the intensity of the green and blue light that reachesthe sensor is much less than that of the red light that reaches thesensor.

Typical embodiments of the inventive VCF sensor group achieve separationof colors by capturing photons in different ranges of depth in a volumeof semiconductor material. FIG. 2 is a vertical doping profile for a VCFsensor group comprising top layer X01 (made of n-type semiconductor),second (p-type) layer X09 below the top layer, third (n-type) layer X02below the second layer, fourth (p-type) layer X10 below the third layer,fifth (n-type) layer X03 below the fourth layer, and p-typesemiconductor substrate X11 below the fifth layer.

FIG. 2A is a cross-sectional view (in a vertical plane) of this VCFsensor group. As shown in FIG. 2A, biasing and readout circuitry iscoupled to layers X01, X02, X03, X04, and X05, and to substrate X11.

Blue, green, and red photodiode sensors are formed by the junctionsbetween the n-type and p-type regions of FIG. 2A, and are disposed atdifferent depths beneath the surface of the semiconductor structure. Thered, green, and blue photocharge signals are all taken from the n-typecathodes (X01, X02, and X03) of three isolated photodiodes.

The readout circuitry of FIG. 2A is of the non-storage type, and issimilar to that described in above-referenced application Ser. No.09/884,863. Readout circuitry for each sensor includes a resettransistor (54 b for the blue sensor, 54 g for the green sensor, and 54r for the red sensor) driven from a RESET signal line and coupledbetween the photodiode cathode and a reset potential (identified asV_(REF) in FIG. 2A), a source-follower amplifier transistor (one oftransistors 56 b, 56 g, and 56 r) whose gate is coupled to thephotodiode cathode and whose drain is maintained at potential V_(SFD)during operation, and a row-select transistor (one of transistors 58 b,58 g, and 58 r) driven from a ROW-SELECT signal line and coupled betweenthe source of the relevant source follower amplifier transistor and arow line. The suffixes “r,” “g,” and “b” are used to denote thewavelength band (red, green, or blue) associated with each transistor.As is known in the art, the RESET signal is active to reset the pixeland is then inactive during exposure, after which the row select line isactivated to read out the detected signal.

Each of p-type regions X09, X10, and X11 is held at ground potentialduring operation. Each of n-type layers X01, X02, and X03 is acarrier-collection element having a contact portion accessible to (andthat can be coupled to) the biasing and readout circuitry. Before eachreadout of the sensor group, the biasing circuitry resets each of then-type layers to the reset potential (above ground potential). Duringexposure to radiation to be sensed, the reversed-biased pairs ofadjacent p-type and n-type layers function as photodiodes: a firstphotodiode whose cathode is layer X01 and whose anode is layer X09; asecond photodiode whose cathode is layer X02 and whose anodes are layersX09 and X10; and a third photodiode whose cathode is layer X03 and whoseanodes are layers X10 and X11. As shown in FIG. 2, each of the n-typelayers X01, X02, and X03 is coupled to biasing and readout circuitry andthus serves as a photodiode terminal.

During typical operation when the photodiodes of FIG. 2 are reversebiased, depletion regions are formed which encompass the majority of thesilicon in which photons are absorbed. In FIG. 2, the depletion regionfor the first photodiode (which senses primarily blue light) is labeled“X04,” the depletion regions for the second photodiode (which sensesprimarily green light) are labeled “X05” and “X06,” and the depletionregions for the third photodiode (which senses primarily red light) arelabeled “X07” and “X08.” The fields within the depletion regionsseparate the electron hole pairs formed by the absorption of photons.This leaves charge on the cathode of each photodiode, and readoutcircuitry coupled to each cathode converts this charge into anelectrical signal. The charge on the cathode of each photodiode isproportional to the number of photons absorbed by the photodiode. Thisproportionality is the quantum efficiency, QE.

FIG. 3 shows the same curves shown in FIG. 1 (indicative of theabsorption of blue, green, and red photons by silicon) and also includeslines indicating the extent of the carrier-collection elements (X01,X02, and X03) and depletion regions of the FIG. 2 structure. Thus, theregion labeled “X01+X04” in FIG. 3 represents the region of FIG. 2 abovethe lower surface of depletion region X04, the region labeled“X05+X02+X06” in FIG. 3 represents the region of FIG. 2 between theupper surface of depletion region X05 and the lower surface of depletionregion X06, and the region labeled “X07+X03+X08” in FIG. 3 representsthe region of FIG. 2 between the upper surface of depletion region X07and the lower surface of depletion region X08. FIG. 3 thus illustratesthe three distinct “sensor” regions in which the three photodiodes ofFIG. 2 absorb photons and in which charge resulting from such absorptionremains (and does not migrate outside the sensor region in which it isproduced) and can be measured by readout circuitry. It should berecognized, however, that electron-hole pairs created between the threesensor regions (e.g., electron-hole pairs created in layer X09 betweenthe lower surface of depletion region X04 and the upper surface ofdepletion region X05) can still diffuse (with high efficiency) into thesensor regions and create charge on the photodiodes that can be measuredby readout circuitry.

The selective absorption of photons by wavelength determines the photoresponse of the three photodiodes. If one considers the position of thesensor regions (“X01+X04,” “X05+X02+X06,” and “X07+X03+X08”) in relationto the curves of FIG. 3 for 450 nm, 550 nm and 650 nm photons, one willsee that the depth and extent of the sensor regions determines thespectral response. In the “X01+X04” region, much more incident bluelight is absorbed than incident green and red light, but some smallamount of green and red light is absorbed. In the “X01+X04” region muchless incident green light is absorbed than incident blue light, and muchmore incident green light is absorbed than incident red light. In the“X05+X02+X06” region, more incident green light is absorbed thanincident blue light (since most of the blue light incident at region“X01+X04” is absorbed in that region and does not reach region“X05+X02+X06”), and more incident green light is absorbed than incidentred light (even though only a small amount of the red light incident atregion “X01+X04” is absorbed in that region so that most such red lightreaches region “X05+X02+X06”).

The full range of incident wavelengths (not just the three wavelengths450 nm, 550 nm and 650 nm) determines the spectral response of the threephotodiodes of FIG. 2, which is similar to that shown in FIG. 4. CurveC1 in FIG. 4 is the spectral response of a top (“blue”) photodiodesimilar to the top (“blue”) photodiode of FIG. 2, curve C2 in FIG. 4 isthe spectral response of a middle (“green”) photodiode similar to themiddle (“green”) photodiode of FIG. 2, and curve C3 in FIG. 4 is thespectral response of a bottom (“red”) photodiode similar to the bottom(“red”) photodiode of FIG. 2.

In an important class of embodiments (including the VCF sensor group ofFIG. 2), the inventive VCF sensor group implements three photodiodes.Such VCF sensor groups are well suited for use in a DSC or digital videocamera. However, in other embodiments, the inventive VCF sensor groupimplements two (or more than three) photodiodes placed at differentdepths within a volume consisting at least mainly of semiconductormaterial.

As noted, materials whose absorptivity varies with wavelength change thespectral content of radiation that propagates through them as a functionof depth into the material. Such materials can have multiple functionsin VCF sensor groups: they can function as filters and also as sensors(or elements of sensors). For example, in the FIG. 2 embodiment each ofthe silicon regions X01, X02, X03, X09, X10, and X11 functions as afilter and also as an element of at least one sensor. In otherembodiments, other semiconductors (or layers of at least two differentsemiconducting materials) similarly function both as sensors (orelements of sensors) and as filters.

In a class of embodiments, the inventive vertical color filter (“VCF”)sensor group includes vertically stacked sensors, the sensors include atop sensor having a top surface, and radiation to be sensed is incidentat the top surface and propagates into the top sensor (through the topsurface) before reaching any other sensor of the group. The top surfacedefines a normal axis (and is typically at least substantially planar).Preferably, the sensors are configured such that when radiationpropagating along a vertical axis of the group (defined above) isincident at the group, the radiation is incident at the top sensor withan incidence angle of less than about 30 degrees with respect to thenormal axis.

Next, with reference to FIG. 5, 6, and 7, we describe embodiments inwhich semiconductor materials other than silicon (e.g., InGaN or otherIII-V semiconductor materials, or semiconductor materials other thansilicon that are not III-V materials) are used to form a VCF sensorgroup. One such semiconductor material that is neither silicon nor aIII-V material is silicon carbide. FIG. 5 is a simplifiedcross-sectional view (in a vertical plane) of a VCF sensor groupincluding top sensor 10, bottom sensor 14, and middle sensor 12positioned between sensors 10 and 14. Each of sensors 10 and 12 consistsof In_(x)Ga_(1-x)N semiconductor material, where x=0.475 for sensor 10and x=0.825 for sensor 12. Sensor 14 consists essentially of silicon.Typically, each of sensors 10 and 12 consists of multiple layers ofIn_(x)Ga_(1-x)N semiconductor that determine at least one junction thatis biased during operation to function as a photodiode, and sensor 14consists of multiple layers of silicon having different doping (e.g., alayer of n-type silicon, and adjacent portions of p-type silicon layersabove and below the n-type layer) that are biased during operation tofunction as a photodiode.

It is within the scope of the invention to employ sensors that consistessentially of one or more III-V semiconductor materials, and determinejunctions (of any kind, including heterojunctions and Schottky barriers)that are biased during operation to function as photodiodes.

FIG. 6 is a table that lists the bandgap energy (in the center columnlabeled “Energy gap”) in electron volts of In_(x)Ga_(1-x)N semiconductorhaving different levels of Indium content (different values of thesubscript “x”). FIG. 6 also lists (in the right column) the opticalwavelength corresponding to each bandgap energy. Thus, FIG. 6 indicatesthat the maximum wavelength that can be absorbed by a sensor made ofIn_(0.1)Ga_(0.9)N semiconductor is 388 nm, that the maximum wavelengththat can be absorbed by sensor 10 of FIG. 5 (made ofIn_(0.475)Ga_(0.525)N semiconductor) is about 500 nm, and that themaximum wavelength that can be absorbed by sensor 12 of FIG. 5 (made ofIn_(0.825)Ga_(0.175)N semiconductor) is about 612 nm.

Thus, sensor 10 transmits all (or substantially all) the green and redlight incident thereon and preferably has thickness sufficient for it toabsorb all (or substantially all) blue light incident on the FIG. 5sensor group. Similarly, sensor 12 transmits all (or substantially all)the red light incident thereon and preferably has thickness sufficientfor it to absorb all (or substantially all) green light incident on theFIG. 5 sensor group. Sensor 14 preferably has thickness sufficient forit to absorb all (or at least a significant amount of the red lightincident thereon.

In general, when using In_(x)Ga_(1-x)N semiconductor material (or otherIII-V semiconductor material) to form a VCF sensor group, the parametersof the material (e.g., the parameter “x” in In_(x)Ga_(1-x)N) are chosento achieve the desired band gap energy for each sensor of the VCF sensorgroup (e.g., so as to make one sensor transparent to light havingwavelength greater than a threshold, where the threshold is determinedby the band gap energy).

More generally, in some preferred embodiments of the invention, at leastone semiconductor material other than silicon is employed to implementat least one sensor of a VCF sensor group, and the material is chosen tomake different sensors of the group selectively sensitive to differentwavelength bands. In some such preferred embodiments, at least twodifferent types of semiconductor materials are employed to implementsensors of a VCF sensor group, and the materials are chosen to makedifferent sensors of the group selectively sensitive to differentwavelength bands.

Some embodiments of the inventive VCF sensor group include at least one“avalanche” photodiode, which is a photodiode that collects more thanone electron per absorbed photon as a result of an “avalanche” gainprocess. In an avalanche gain process, a first electron-hole pairgenerated by absorption of a photon generates at least one additionalelectron-hole pair, assuming that the energy of the electron of thefirst electron-hole pair exceeds the bandgap energy of semiconductormaterial that forms the photodiode sensor. A semiconductor material hasan ionization coefficient (a_(n)) for electrons and an ionizationcoefficient (a_(p)) for holes, where 1/a_(n) is the average distanceover which an electron is accelerated in the material before it createsan electron/hole pair by impact ionization, and 1/a_(p) is the averagedistance over which a hole is accelerated in the material before itcreates an electron/hole pair by impact ionization. It is much moredifficult to implement a practical avalanche photodiode when thephotodiode is formed of semiconductor material in which the ratio ofionization coefficients, a_(p)/a_(n), is nearly equal to one than in thecase that the photodiode is formed of semiconductor material in whichthe ratio of ionization coefficients, a_(p)/a_(n), is much greater thanone or much less than one.

In some embodiments of the invention, at least one sensor of a VCFsensor group is an avalanche sensor that includes an optical absorptionregion and an avalanche region separate from the optical absorptionregion. For example, FIG. 7 is a cross-sectional view of such anavalanche sensor that can be included in a VCF sensor group. The sensorof FIG. 7 comprises substrate 20 (made of n+silicon), layer 21 (made ofn−silicon) on substrate 20, layer 22 (made of n-type In_(x)Ga_(1-x)Nsemiconductor material having a relatively low dopant concentration) onlayer 22, and layer 23 (made of p-type In_(x)Ga_(1-x)N semiconductormaterial having a relatively high dopant concentration) on layer 23.Metal contact 27 is formed on layer 23, and substrate 20 is coupled tometal contact 25 by a vertically oriented contact region consisting ofn+silicon. In operation, bias voltage is applied across metal contacts25 and 27, and readout circuitry can be coupled to contact 27. Isolationis provided by dielectric material 27A (which can consist ofphotoresist, e.g., polymethylglutarimide resist) around layers 21, 22,and 23, and dielectric material 24 (which can be silicon nitride)between layers 21, 22, and 23 and dielectric material 24 (and betweensubstrate 20 and material 24).

In operation, layers 22 and 23 function as an optical absorption regionin which electron-hole pairs are formed in response to incident photons.The In_(x)Ga_(1-x)N semiconductor material that forms layers 22 and 23has a ratio of ionization coefficients (a_(p)/a_(n)) that is muchgreater (or much less) than one, and thus layers 22 and 23 are notutilized as an avalanche gain region.

In operation, layers 21 and 20 function as an avalanche gain region inwhich electron-hole pairs are formed in response to electron-hole pairsformed in the optical absorption region. The silicon that forms layers22 and 23 has a ratio of ionization coefficients (a_(p)/a_(n)) that ismuch closer to one than is the ratio of ionization coefficients forlayers 22 and 23.

In general, some embodiments of the inventive VCF sensor group includeat least one sensor that is an avalanche photodiode, wherein theavalanche photodiode includes an optical absorption region made ofsemiconductor material (e.g., InGaN) whose ionization coefficient forelectrons is very different than its ionization coefficient for holes,and an avalanche region separate from the optical absorption region madeof another semiconductor material (e.g., silicon) having more nearlyequal ionization coefficients for electrons and holes. It iscontemplated that one important use of a sensor implemented as anavalanche photodiode is to sense radiation of low intensity, such asradiation that has had its intensity significantly reduced (e.g., byabsorption) during propagation through at least one filter and/or atleast one other sensor before reaching the avalanche photodiode.

In other embodiments of the inventive VCF sensor group, at least onefilter that does not function as a sensor (or sensor element) is stackedwith at least one layer of semiconductor material that functions as asensor (or as an element or one or more sensors). Such a filter can, butneed not, have the same spectral sensitivity as does the silicon in theFIG. 2 embodiment.

Filters remove wavelengths from radiation in the following sense. Foreach filter, there are first and second wavelengths such that, if thefirst and second wavelengths are incident at the filter with intensities“I1” and “I2,” respectively, and the transmitted intensities of thefirst and second wavelengths (after transmission through the filter) are“O1” and “O2,” respectively, then O1≦I1, O2≦I2, and O1/O2<I1/I2.

One type of filter that is included in some embodiments of the inventiveVCF sensor group is a “conversion filter” (e.g., a “conversion layer”)that changes the wavelengths of electromagnetic radiation that isincident thereon. A “conversion” filter absorbs photons of onewavelength and emits photons at at least one shorter or longerwavelength. Typically, the material that comprises a conversion filteris a non-linear optical material. A conversion filter can be used toconvert photons with frequencies below a sensor cutoff frequency tohigher frequencies so that they can be detected. Alternatively, aconversion filter can be used to convert photons with frequencies abovea threshold frequency to lower frequencies so that they can be detected.An example of the latter is the X-ray conversion layer used to convertX-rays, which easily penetrate most detecting materials, to visiblelight which is easily detected. Either a layer of Gadolinium Oxy-sulfidehaving thickness of about 100 μm, or a layer of Cesium Iodide doped withThallium having thickness in the range from about 100 μm to 600 μm couldbe used as such an X-ray conversion layer in some embodiments of theinvention.

There are two related ways of detecting photons in a group of spectralbands, and each can be used to implement the invention. In someembodiments of the inventive VCF sensor group, at least one filterremoves photons outside at least one wavelength band and at least twovertically stacked sensors detect remaining photons, where each sensoris an element distinct from each filter. Other embodiments of theinventive VCF sensor group do not include a non-sensor filter (a filterthat is not a sensor), but do include sensors that are sensitive tolimited wavelength bands. Other embodiments of the invention implementcombinations of these approaches, for example by including a firstsensor and a second sensor below the first sensor, where the firstsensor absorbs a limited range of wavelengths and passes photons outsidethis range to the second sensor, and the second sensor is sensitive toall wavelengths. In the example, the first sensor functions as a filterfor the second sensor.

In some embodiments of the invention, at least one non-sensor filter ispositioned between at least one pair of vertically stacked sensors of aVCF sensor group, or above the top sensor of the group, or below thebottom sensor of the group. When such a filter is positioned between apair of vertically stacked sensors of a VCF sensor group, the filter canbe of any of a variety of different types, including (but not limited tothe following): the filter can absorb radiation in one wavelength bandand transmit other wavelengths without reflecting significant radiationof any wavelength; the filter can reflect radiation in one wavelengthband and transmit other wavelengths without absorbing significantradiation of any wavelength; or the filter can be highly transmissive toradiation in one wavelength band, absorptive of radiation in anotherwavelength band, and reflective of radiation in a third wavelength band.The VCF sensor group of FIG. 8 includes two non-sensor filters of thelatter type: color filter 43 and color filter 48. It should beappreciated that the FIG. 8 sensor group is only one example of the manyembodiments of the invention that are contemplated.

FIG. 8 is a cross-sectional view (in a vertical plane) of a portion ofone embodiment of an array of the inventive VCF sensor groups whichincludes two non-sensor filters (layers 43 and 48) and four insulationlayers (diffusion barriers 42, 44, 47, and 48). Each insulation layercan consist of silicon dioxide. In FIG. 8, one VCF sensor groupcomprises layer 51 (made of n-type semiconductor) and layers of p-typesemiconductor material 50 above and below layer 51, insulating layer 49below material 50, color filter 48 below layer 49, insulating layer 47below filter 48, layer 46 (made of n-type semiconductor) and layers ofp-type semiconductor material 45 above and below layer 46, insulatinglayer 44 below material 45, color filter 43 below layer 44, insulatinglayer 42 below filter 43, and layer 41 (made of n-type semiconductor)and p-type semiconductor substrate material 40 above and below layer 41.Vertically oriented plug contacts connect each of layers 41, 46, and 51to the sensor group's top surface, so that each of layers 41, 46, and 51can be coupled to biasing and readout circuitry. Light shield 54 ismounted above the plug contacts to prevent radiation (normally incidentat the sensor groups' top surface) from reaching the plug contacts,which would reduce frequency selectivity. The array of FIG. 8 alsoincludes a second VCF sensor group comprising layer 63 (made of n-typesemiconductor) and layers of the p-type semiconductor material 50 aboveand below layer 63, insulating layer 49 below material 50, color filter48 below layer 49, insulating layer 47 below filter 48, layer 62 (madeof n-type semiconductor) and layers of the p-type semiconductor material45 above and below layer 62, insulating layer 44 below material 45,color filter 43 below layer 44, insulating layer 42 below filter 43, andlayer 61 (made of n-type semiconductor) and p-type semiconductorsubstrate material 40 above and below layer 61. Vertically oriented plugcontacts connect each of layers 61, 62, and 63 to the sensor group's topsurface, so that each of layers 61, 62, and 63 can be coupled to biasingand readout circuitry. Light shield 53 is mounted above the plugcontacts of the second VCF sensor group to prevent radiation (normallyincident at the sensor groups' top surface) from reaching the plugcontacts.

In variations on the FIG. 8 embodiment, horizontally oriented variationson n-type layers 51 and 63 (which lack vertically oriented contactportions) are exposed at the top surface of the sensor group (and arenot covered by semiconductor material 50). Each such exposed, n-typelayer can be directly connected (e.g., by a metal contact formedthereon) to biasing and readout circuitry. Similarly, in variations onthe FIG. 8 embodiment, n-type layers 46 and 62 lie directly under layer47 (and are not separated from layer 47 by p-type semiconductor material45) and n-type layers 41 and 61 lie directly under layer 42 (and are notseparated from layer 42 by p-type semiconductor material 40).

Each of the p-type semiconductor layers of FIG. 8 is held at groundpotential during operation. Each of the n-type layers is coupled by aplug contact that is accessible to (and can be coupled to) biasing andreadout circuitry. Before each readout of each sensor group, biasingcircuitry resets each of the n-type layers to a reference potential(above ground potential). During exposure to radiation to be sensed, thereversed-biased pairs of adjacent p-type and n-type layers of the firstsensor group function as photodiodes: a first photodiode whose cathodeis layer 51 and whose anodes are the adjacent layers of material 50(referred to as a “blue” sensor since it absorbs more blue photons thangreen or red photons in response to white light incident at the top ofthe sensor group); a second photodiode whose cathode is layer 46 andwhose anodes are the adjacent layers of material 45 (referred to as a“green” sensor since it absorbs more green than blue or red photons whenwhite light is incident at the top of the sensor group); and a thirdphotodiode whose cathode is layer 41 and whose anodes are the adjacentlayers of material 40 (referred to as a “red” sensor since it absorbsmore red than blue or green photons when white light is incident at thetop of the sensor group). During exposure to radiation to be sensed, thereversed-biased pairs of adjacent p-type and n-type layers of the secondsensor group also function as photodiodes: a first photodiode whosecathode is layer 63 and whose anodes are the adjacent layers of material50 (also referred to as a “blue” sensor since it absorbs more bluephotons than green or red photons in response to white light incident atthe top of the second sensor group); a second photodiode whose cathodeis layer 62 and whose anodes are the adjacent layers of material 45(referred to as a “green” sensor since it absorbs more green than blueor red photons when white light is incident at the top of the secondsensor group; and a third photodiode whose cathode is layer 61 and whoseanodes are the adjacent layers of material 40 (referred to as a “red”sensor since it absorbs more red than blue or green photons when whitelight is incident at the top of the second sensor group).

When layers 40, 41, 45, 46, 50, and 51 are made of crystalline silicon(as is typical), layers 51 and 50 are preferably thinner than layers 46and 45, respectively, and layers 41 and 40 are thinner than layers 51and 50, respectively, by amounts sufficient to ensure that the intensityratio of green light to red light incident at each green sensor issufficiently high while ensuring that much more red light than greenlight is incident at each red sensor and that much more blue light thangreen light is absorbed by each blue sensor. Typically, the combinedthickness of layers 51 and 50 in the first sensor group (and layers 63and 50 in the second sensor group) is 0.3 μm or less and the combinedthickness of layers 45 and 46 in the first sensor group (and layers 45and 62 in the second sensor group) is about 0.5 μm.

Color filter 43 is a “red pass/cyan reflect” filter that is highlytransmissive to red light but reflects most or nearly all of the blueand green light incident thereon. Color filter 48 is a “yellow pass/bluereflect” filter that is highly transmissive to red and green lightincident thereon but reflects most or nearly all the blue light incidentthereon. Other embodiments of the invention employ transmissive filtersthat are not reflective.

Filter 43 functions to increase the ratio of red to green light (and theratio of red to blue light) that is absorbed by each red sensor, and canreduce or eliminate red/green discrimination problems that mightotherwise affect the red sensors if filter 43 were omitted. Similarly,filter 48 finctions to increase the ratio of green to blue light that isabsorbed by each green sensor, and can reduce or eliminate green/bluediscrimination problems that might otherwise affect the green sensors iffilter 48 were omitted.

Filter 48 also finctions to increase the ratio of blue to green (andred) light that is absorbed by each blue sensor, since blue lightreflecting from filter 48 has another chance to be absorbed in a bluesensor. Each blue sensor's absorption of blue light is improved withoutincreasing its response to red and green light, since there is no morethan insignificant reflection of red and green light from filter 48 backinto the blue sensors. Similarly, filter 43 also functions to increasethe ratio of green to red light that is absorbed by each green sensor,since green light reflecting from filter 43 has another chance to beabsorbed in a green sensor. Each green sensor's absorption of greenlight is improved without increasing its response to red light, sincethere is no more than insignificant reflection of red light from filter43 back into the green sensors. Very little blue light reaches the greensensors since nearly all the blue light is either absorbed in the bluesensors or reflected back toward the blue sensors by filter 48.

There are a wide variety of materials that may act as a filter in a VCFsensor group (e.g., filter 43 or 48 in FIG. 8, or a filter that isreflective of a wavelength band but transmissive to all otherwavelengths, or a filter that is absorptive of a wavelength band but notreflective). These materials may be used in combination or in variousthicknesses. The arrangements are determined partly by their opticalproperties, but also in good measure by process integrationconsiderations.

Materials and interfaces between materials can reflect photons. When amirror's reflectivity is selective by wavelength, the mirror (either amaterial or an interface between materials) can function as a filter inthe inventive VCF sensor group. For example, some embodiments of theinventive VCF sensor group include a dichroic mirror that both transmitsradiation in a first wavelength band and reflects radiation in a secondwavelength band.

As discussed above, stacked layers of material whose optical absorptionvaries with wavelength can be used as filters in various embodiments ofthe inventive VCF sensor group. In preferred embodiments that includelayers of differently doped semiconductor material (e.g., silicon), atleast one semiconductor layer is used both as a filter and a sensor. Ina VCF sensor group in which a layer of semiconductor material is usedboth as a filter and as a cathode (or anode) of a photodiode sensor, thesensor's spectral sensitivity can be controlled somewhat by controllingthe bias voltage applied across the photodiode's anode(s) and cathode,and can also be controlled by determining the doping levels andlocations of the dopant atoms, and the structure spacing of sensorelements.

Another type of filter that is included in some embodiments of theinvention is a thin metal film. Thin metal films can act as partialreflectors and thereby filter incoming photons. The reflected photonsreturn through any layers above them, which gives them a second chanceto be absorbed.

Other types of filter that are included in some embodiments of theinvention are interference filters (e.g., stacks of layers of dielectricmaterial having differing dielectric constants) that reflect certainwavelengths and pass others, and organic and inorganic dyes andpigments.

In some embodiments of the invention, filters are distributed among VCFsensor groups of an array in any of a variety of patterns, for example,as described in parent application Ser. No. 10/103,304. The filters can,but need not, all be identical. Preferably, each filter is integrallyformed with one of the VCF sensor groups (e.g., as a layer formed on asemiconductor layer or between semiconductor layers). Alternatively, thefilters can be fabricated separately from the sensor groups and thenpositioned over the sensor group array and bonded to (or otherwiseattached or held in a fixed position relative to) the VCF sensor groups.The filters can be provided in an alternating or “checkerboard” manneras shown in FIG. 8A, in which each square labeled “RGB” indicates a VCFsensor group, and each square marked with an “X” indicates a VCF sensorgroup including one of the filters. As shown in FIG. 8A, eachodd-numbered sensor group in each odd-numbered row includes one of thefilters, and each even-numbered sensor group in each even-numbered rowincludes one of the filters, thus obtaining optimal spatial frequencybetween color sensor groups having a filter and color sensor groups nothaving a filter.

Alternatively, the filters can be provided a pattern as shown in FIG.8B, in which each square labeled “RGB” indicates a VCF sensor group, andeach square marked with an “X” indicates a VCF sensor group includingone of the filters. When the filters are provided in the FIG. 8Bpattern, the filters are distributed in a manner that permits bothfull-measured color readout and mosaic emulation readout, whileguaranteeing that both types of image readouts contain every combinationof color sensor group output and color filter. Alternatively, filterscan be distributed among the sensor groups of a VCF sensor group arrayin any of many other patterns, some of which are described in parentapplication Ser. No. 10/103,304.

Some embodiments of the inventive VCF sensor group include at least onelens in instead of or in addition to at least one filter. For example, amicro-lens can be formed over each of all or some of the VCF sensorgroups of a VCF sensor group array. In some cases when metallization (oranother structure) limits the size of the aperture of a VCF sensor group(the area in the imaging plane in which incident radiation willpropagate to at least one sensor), photoresist can be deposited on theaperture and then developed so that the photoresist material melts intoa concave or convex shape thereby forming a micro-lens. Depending on thecharacteristics of the material comprising a lens and the lens shape, alens can function as a filter as well as a lens. For example, FIG. 9 isa cross-sectional view (in a vertical plane) of a portion of a variationthe VCF sensor group array of FIG. 8. The FIG. 9 array includes a firstVCF sensor group including n-type semiconductor layers 51, 46, and 41formed in p-type semiconductor material, vertically-oriented contactsconnecting each of layers 41, 46, and 51 to the sensor group's topsurface, and light shield 54 mounted above the contacts to preventradiation (normally incident at the sensor groups' top surface) fromreaching the contacts. The FIG. 9 array also includes a second VCFsensor group including n-type semiconductor layers 61, 62, and 63 formedin p-type semiconductor material, vertically-oriented contactsconnecting each of layers 61, 62, and 63 to the sensor group's topsurface, and light shield 53 mounted above these contacts to preventradiation (normally incident at the sensor groups' top surface) fromreaching the contacts. Light shields 53 and 54 are formed in layer 64which is transparent to the radiation to be sensed. Light shields 53 and54 surround the first sensor group's aperture, and light shield 53 andanother light shield (not shown) surround the second sensor group'saperture. Convex micro-lens 65 is formed on layer 64 over the firstgroup's aperture and convex micro-lens 66 is formed on layer 64 over thesecond group's aperture.

When micro-lenses are distributed among the sensor groups of a VCFsensor group array in an alternating pattern (such as that shown in FIG.8A), subsets of the sensor groups having different sensitivity toradiation can be selected independently. This provides an expandeddynamic range for the array as a whole.

The aperture of each sensor group of a VCF sensor group array willtypically be square or octagonal but can alternatively have anothershape (e.g., a rectangular, circular, or irregular shape). Themicro-lenses formed over the apertures of all or some sensor groups ofsuch an array will typically be square, but can have other shapes.

Some embodiments of the inventive VCF sensor group include at least onemicro-lens that is a compound lens (e.g., a combination of a concavemicro-lens and a convex micro-lens).

It is well known to form micro-lenses as a top layer of a CCD imagesensor array, with one micro-lens over each sensor of the array. It isalso well known to include micro-lenses as an intermediate layer of aCCD image sensor array, for example with two vertically-separatedmicro-lenses over each sensor of the array and a color filter betweeneach such pair of vertically-separated micro-lenses. In some embodimentsof the present invention, a micro-lens (e.g., micro-lens 65 of FIG. 9)is positioned relative to the sensors of a VCF sensor group so as torefract radiation into the top sensor of the group (e.g., the sensorincluding layer 51 in FIG. 9) so that at least some of the radiationwill propagate through the top sensor and to each of the sensorspositioned below the top sensor, assuming that the radiation includes atleast one wavelength that is neither absorbed in the group nor reflectedby an element of the group before it can reach the bottom sensor.

Materials already used (for other purposes) in semiconductor processingare highly desirable for implementing filters, lenses, and sensors intypical embodiments of the invention because they can be added to a VCFsensor group without modifying the process. Examples of such a materialare polysilicon, silicon dioxide, and silicon nitride. A layer ofpolysilicon can be used as a filter whose absorption spectrum depends onits crystalline characteristics and conductivity and the layer'sthickness and depth relative to other elements of a VCF sensor group.Layers of silicon dioxide and silicon nitride grown on a surface (e.g.,a silicon surface) can form an interference filter in a VCF sensorgroup.

The expression “minimum-sized” carrier-collection element of a VCFsensor group that embodies the invention is used herein to denote eachcarrier-collection element of the group whose projection, on a planeperpendicular to a normal axis defined by a top surface of a top sensorof the group, has an area that is not greater than the projected area ofeach other carrier-collection element of the group on such plane. Theexpression “minimum collection area” (of a group) is used herein todenote the projected area of a minimum-sized carrier-collection elementof the group, on a plane perpendicular to a normal axis defined by a topsurface of a top sensor of the group. In a class of embodiments of theinventive sensor group, the carrier-collection element of one sensor ofthe group has substantially larger “size” (area projected in a planeperpendicular to a normal axis of a top surface of a top sensor of thegroup) than does each minimum-sized carrier-collection element of thegroup, as in the sensor groups of FIGS. 10, 10A, and 10B. In preferredembodiments in this class, one carrier-collection element of a sensorgroup has size that is at least twice the group's minimum collectionarea. This carrier-collection element is typically shared by at leastone other sensor group of an array, and its size is typically at leastsubstantially equal to the sum of the sizes of all the groups that shareit.

The array of FIG. 10 includes a plurality of sensor groups, six of whichare shown in FIG. 10. Each sensor group includes one green sensor (whosecarrier-collection area is not shared with any other sensor group), oneblue sensor (shared with one other sensor group), and one red sensor(shared with one other sensor group. The carrier-collection area of eachred sensor and each blue sensor is shared by two sensor groups. Thecarrier-collection areas for blue and red photons are larger than thecollection areas for green photons.

In variations on the FIG. 10 or 10B array, at least onecarrier-collection area (shared by two sensor groups) comprises two ormore portions that are initially formed to be laterally separated fromeach other and are then shorted together to form a single effectivecarrier-collection area. For example, each blue sensor can include twolaterally separated carrier-collection areas for blue photons, eachformed over a different carrier-collection area for green photons, withthe two carrier-collection areas for blue photons being laterallyseparated to provide space for forming at least one transistor on thearray's top surface therebetween. The two laterally separatedcarrier-collection areas of each blue sensor are shorted together toform a single effective carrier-collection area for blue photons thathas larger total size than each of the array's carrier-collection areasfor green photons.

With reference again to FIG. 10, the electric charge collected on eachred sensor is converted to an electrical signal indicative of twice theaverage of the incident red intensity at the two sensor groups whichshare the red sensor. The electric charge collected on each blue sensoris converted to an electrical signal indicative of twice the average ofthe incident blue intensity at the two sensor groups which share theblue sensor. Thus, the array's resolution with respect to green light istwice its resolution with respect to red or blue light. This type ofarray increases the signal to noise ratio in the blue and red channelswhile maintaining high spatial resolution in a green (or luminance-like)channel. The high luminance resolution is achieved because every pixellocation has an active green sensor, in contrast with conventional imagesensor arrays using the Bayer pattern that have a green sensor at onlyhalf of the pixel locations. Those of ordinary skill in the art willrecognize that maintaining high luminance resolution via a highersampling rate in the green channel will reduce the presence of aliasingartifacts in interpolated images generated with such an array. Largerblue and red carrier-collection areas further reduce the presence ofaliasing artifacts.

In other embodiments, the carrier-collecting areas of the blue sensorsof an array of VCF sensor groups are smaller than the carrier-collectingareas of the red and green sensors of the array.

In some embodiments of an array of VCF sensor groups, one sensor groupincludes at least one sensor (or element of a sensor) that is sharedwith another sensor group. FIG. 10A is a cross-sectional view (in avertical plane) of such an array. In FIG. 10A, a first sensor groupcomprises a first sensor which in turn comprises layer 102 (made ofn-type semiconductor) and the regions of p-type material 100 immediatelyabove and below layer 102, and a second sensor which in turn compriseslayer 101 (made of n-type semiconductor) and the regions of p-typematerial 100 immediately above and below layer 101. FIG. 10A also showsa second sensor group comprising a third sensor (which in turn compriseslayer 103 made of n-type semiconductor and the regions of p-typematerial 100 immediately above and below layer 103) and the secondsensor. Thus, the second sensor (which includes layer 101) is shared bythe two sensor groups, and each of the separate first and third sensorsis positioned at the same vertical level in the array.

The FIG. 10A array could be configured so that the first sensor's outputis indicative of a blue component of a first pixel, the third sensor'soutput is indicative of a blue component of a second pixel, and thesecond sensor's output is indicative of a green component of both thefirst pixel and the second pixel. The FIG. 10A array is preferablyoperable in a mode in which it has better resolution with respect togreen light than blue light (e.g., by using the outputs of the first,second, and third sensors separately), and in another mode in which ithas equal resolution with respect to blue light and green light (e.g.,by averaging the outputs of the first and third sensors, and using thisaveraged value with the output of the second sensor). The FIG. 10A arrayis a simple embodiment with sensors at only two depths. The sensorgroups of other embodiments of the inventive array have sensors arrangedvertically at three or more different depths.

In the VCF sensor group array of FIG. 10B, the carrier-collectionelements of the red and blue sensors of each sensor group have largersize than does the carrier-collection element of the group's greensensor. The array of FIG. 10B includes a plurality of sensor groups,four of which are shown in FIG. 10B. Each sensor group includes onegreen sensor whose carrier-collection area (182, 183, 184, or 185) isnot shared with any other sensor group, one blue sensor whosecarrier-collection area (180) is shared with each of three other sensorgroups, and one red sensor whose carrier-collection area (181) is sharedwith each of three other sensor groups. The carrier-collection areas forblue and red photons are larger than the collection areas for greenphotons. The electric charge collected on each red sensor (due to photonabsorption) is converted to an electrical signal (typically a voltage)indicative of the average of the incident red intensity at the foursensor groups that share the red sensor. The electric charge collectedon each blue sensor is converted to an electrical signal (typically avoltage) indicative of the average of the incident blue intensity at thefour sensor groups that share the blue sensor. Typically, voltageoutputs of the red and blue sensors of the FIG. 10B array (andvariations on such array) do not need to be scaled relative to voltageoutputs of the green sensors, since the increase in the electric chargecollected on each sensor due to an increase in the sensor'scarrier-collection area is proportional to the increase in the sensor'scapacitance due to such carrier-collection area increase.

When fabricating an array of VCF sensor groups so that the output ofeach sensor group determines a pixel, it is necessary to isolate thesensor groups from each other to avoid cross talk between pixels. Ifelectrons and holes created in one sensor group can drift into another,the resolution of the imager will be reduced. In preferred embodimentsof the invention, such isolation is accomplished by fabricating sensorgroups whose physical design contains the charge generated within them.

For example, with reference again to FIG. 10A, in the array of FIG. 10Athe lower and larger “second sensor” (including layer 101) is isolatedfrom its neighbor (partially shown but not labeled) at the same verticallevel by an n-p substrate junction just as the smaller “first” and“third” sensors (including layers 102 and 103 respectively) are isolatedfrom each other by an n-p substrate junction.

Some conventional sensor arrays do not implement such isolation betweenthe sensors whose output determines different pixels. For example, onetype of conventional sensor array, shown in FIG. 11 and described inBartek, Sensors and Actuators A, 41-42 (1994), pp. 123-128, comprisesphotodiode sensors (e.g., photodiode 30) created in a layer (31) ofepitaxial silicon (epi) which is in common for all the pixels. In thisconfiguration, charge created in one sensor can drift into a neighboringsensor or possibly farther. Because the structure in FIG. 11 lacksisolation regions (e.g., p-type semiconductor regions) between sensors,the common epi layer (layer 31) provides a path which can conductcarriers from under one pixel to under another.

Various methods can be used to isolate the sensors in a VCF sensor groupfrom each other, or to isolate sensor groups (pixels) from each other inVCF sensor group array that embodies the invention. Process integrationis an important factor in determining the method used. One method thatcan be used is junction isolation, which is commonly used to isolatetransistors in silicon-based processes. The junction must be able towithstand sufficient voltage across it to prevent leakage. There may beenough doping in the substrate or an epi layer to provide adequatejunction isolation, or an increased doping between neighboring regionsto be isolated from each other (e.g., neighboring VCF sensor groups) maybe required to implement junction isolation. This increased doping canbe produced using the “field implant” techniques employed to isolateneighboring transistors in a MOS process.

Other embodiments of the inventive VCF sensor group and array of VCFsensor groups employ dielectric isolation which places an insulatingmaterial between semiconducting regions. This can be done by fabricatingeach sensor group in a block of semiconducting material with an oxidelayer under the sensor group. There are a variety of methods forcreating such a structure, such as growing silicon on sapphire,implanting a layer of oxygen through the upper layers of a silicon waferand reacting the oxygen with the silicon to form an oxide layer, andremoving a layer of processed silicon from a wafer and transferring itto an insulating substrate.

Dielectric isolation can be used to isolate semiconductor sensor groupsfrom each other in an array of VCF sensor groups. When the sensor groupsare displaced laterally from each other and formed in a volume ofsemiconductor material, such isolation can be implemented by forming thegroups on top of an insulating layer, etching a trench in the volume ofsemiconductor material, and growing or depositing an insulator in thetrench. More generally, a trench filled and/or lined with at least oneof an insulator and semiconductor material that is doped and biased(during operation) to provide field isolation (e.g., a trench, linedwith semiconductor material more heavily doped than the bulksemiconductor material between adjacent structures to be isolated fromeach other to passivate leakage, and then filled with an oxide or otherinsulating material) are used to isolate inventive VCF sensor groupsfrom each other in some embodiments of the invention. Use of suchtrenches (to isolate conventional CMOS structures) is termed “trenchisolation” in CMOS technology. Trench isolation can be applied toisolate VCF sensor groups from each other in typical embodiments of theinvention because trenches can be etched deep enough to separate VCFsensor groups which are several micrometers deep, such as those createdin a typical array of silicon based VCF sensor groups.

An example of a combination of dielectric isolation (implemented bytrench isolation) and junction isolation is shown in FIG. 12. In FIG.12, a first VCF sensor group includes vertically-separated n-typesemiconductor layers 151, 152, and 153 (e.g., silicon) formed in p-typesemiconductor material 150 (which can be silicon). Contact 154 isprovided for coupling p-type material 150 to biasing circuitry.Vertically oriented plug contacts connect each of layers 151 and 152 tothe sensor group's top surface, so that each layer can be coupled tobiasing and readout circuitry. The plug contacts can be formed asdescribed in above-cited U.S. application Ser. No. 09/884,863. A secondVCF sensor group includes vertically-separated n-type semiconductorlayers 161 and 162 that are also formed in p-type semiconductor material150. Each sensor of the first sensor group is isolated from the secondsensor group by an n-p substrate junction, just as the sensors in thefirst sensor group are isolated from each other by n-p substratejunctions. Lateral isolation between the first and second sensor groupsis accomplished by trench isolation, namely by trench 157 lined withinsulating material 158 (which can be silicon dioxide or siliconnitride, for example) formed between them. Trench 155 (lined with oxide156) isolates the first sensor group from a third sensor group (notshown in FIG. 12) adjacent to the first sensor group. Insulator layer148 (which can be silicon dioxide or silicon nitride, for example) belowthe bottom sensor of each VCF sensor group also functions to isolate thesensor groups from each other.

The trenches employed in accordance with the invention for trenchisolation between VCF sensor groups can be shallow trenches with a lowaspect ratio (e.g., trenches having quarter-micron depth of the typeconventionally used in some CMOS integrated circuits). Typically,however, the trenches employed in accordance with the invention fortrench isolation between VCF sensor groups will be deeper trenches witha high aspect ratio (e.g., trenches of the type conventionally used insome DRAM integrated circuits).

With reference to FIGS. 20-25, we next describe an improved techniquefor providing buried layer isolation that is employed in preferredembodiments of the inventive VCF sensor group. During operation of eachsuch embodiment of the inventive VCF sensor group, there is a“non-collecting” volume of semiconductor material of a first type(either p-type or n-type) between each two “carrier-collecting” sensorregions (of the opposite semiconductor type). Carriers (electrons orholes) can be photogenerated in a non-collecting volume of a sensorgroup. Carriers that have been photogenerated in a carrier-collectingregion, or that have migrated to a carrier-collecting region after beingphotogenerated elsewhere, can be collected by readout circuitry. In somecases, carriers that have been photogenerated in a non-collecting volumeof a sensor group can migrate to a carrier-collecting sensor region of aneighboring sensor group. Typically, photogenerated carriers can migratefrom a non-collecting volume to any of at least two carrier-collectingsensor regions (in one sensor group or in different sensor groups),although barriers (e.g., barrier 205 of FIG. 20 to be described below)can be formed in accordance with the invention to discourage suchmigration in undesired directions.

As shown in FIG. 20, a sensor group can include upper carrier-collectingsensor region (including photodiode cathode 200 comprising n-typesemiconductor material), lower carrier-collecting sensor region(including photodiode cathode 202 comprising n-type semiconductormaterial), non-collecting photodiode anode layers 201 and 203(comprising grounded p-type semiconductor material) between sensorregions 200 and 202, and non-collecting photodiode anode layer 204(comprising grounded p-type semiconductor material) below sensor region202.

In order to provide isolation between each pair of vertically-separatedsensors of the sensor group, a blanket barrier layer of more heavilydoped semiconductor material of the first type is laminated betweenupper and lower portions of each non-collecting volume (and thus betweenthe sensors). Here, we use the term “laminated” in a broad sense thatdoes not imply that any specific method (e.g., physical bonding ofseparate structures, or an implantation process) is used to form theblanket barrier layer. For example, as shown in FIG. 20, the sensorgroup includes blanket barrier layer 205 (comprising p-typesemiconductor material) between layers 201 and 203 of p-material (andthus between the carrier-collecting sensor regions comprising cathodes200 and 202). The upper carrier-collecting sensor region (comprisingcathode 200) can be a “blue” sensor, the lower carrier-collecting sensorregion (comprising cathode 202) can be a “green” sensor, and the groupcan also include a “red” sensor (not shown) below layer 204, and asecond blanket barrier layer of p-type material between layer 204 andthe red sensor.

FIG. 21 is a graph of dopant concentration as a function of depth in thesensor group of FIG. 20, which shows the locations of cathode layers 200and 202 and barrier 205. During operation of the FIG. 20 sensor group,the presence of barrier 205 results in the electric potential having agradient which directs photogenerated electrons to the nearest one ofcathode layers 200 and 202 so that they do not drift in an undesireddirection (e.g., from a point close to cathode layer 200 all the way tocathode 202 or to the cathode of an adjacent sensor group). By virtue ofits location, barrier 205 also reduces the capacitance of the FIG. 20sensors below the capacitance of the sensors of the sensor group ofbelow-described FIG. 22.

Positioning of blanket barrier layers in accordance with the presentinvention between vertically stacked carrier-collecting sensor regions(as in FIG. 20) contrasts with positioning of blanket barriers at (or atand slightly below) the same vertical level as each carrier-collectingsensor region, as shown in FIG. 22 and described in above-referencedU.S. application Ser. No. 09/884,863. application Ser. No. 09/884,863teaches that each blanket barrier (e.g., each of layers 206 and 207 ofp-type semiconductor material shown in FIG. 22) is implanted across anentire wafer (in which an array of VCF sensor groups is to be formed),and carrier-collecting sensor regions (e.g., those including cathodes200 and 202 of n-type semiconductor material shown in FIG. 22) are thenformed by implantation on selected areas of each blanket barrier toproduce sensors for different sensor groups of the array. Each blanketbarrier produced by the prior technique (disclosed in application Ser.No. 09/884,863) and the present invention is intended to preventcarriers produced in a non-collecting volume from leaking vertically tocarrier-collecting sensor regions (other than the nearestcarrier-collecting sensor region) of the same sensor group and fromleaking horizontally to a non-collecting volume of another sensor groupand then vertically to a carrier-collecting sensor region of the othersensor group. Examples of “non-collecting volumes” are the portion ofanode layer 201 (comprising p-semiconductor material) of FIG. 22 midwaybetween cathodes 200 and 202, a portion of anode layer 201 of FIG. 20that is very near to barrier 205 but relatively far from cathode 200,and a portion of anode layer 203 of FIG. 20 that is very near to barrier205 but relatively far from cathode 202.

FIG. 23 is a graph of dopant concentration as a function of depth in thesensor group of FIG. 22, which shows the locations of cathode layers 200and 202 and barriers 206 and 207. During operation of the FIG. 22 sensorgroup, the presence of barriers 206 and 207 results in the electricpotential having a gradient which allows electrons photogenerated inlayer 201 to drift in undesired directions (e.g., from a point close tocathode layer 200 all the way to cathode 202 or to the cathode of anadjacent sensor group). By virtue of their locations, barriers 206 and207 also increase the capacitance of the FIG. 22 sensors above thecapacitance of the sensors of above-described FIG. 20.

The inventive technique for positioning and forming blanket barriers hasseveral advantages, including that it reduces photodiode capacitance(thus increasing the output voltage of each photodiode and reducing thetime required to reset each photodiode between exposures), and reduces(beyond the level attainable by the prior technique) the leakage ofphotogenerated carriers to the wrong carrier-collecting region of asensor group or to an adjacent sensor group. During operation, thepotential gradient produced in accordance with the invention betweenvertically-separated carrier-collecting regions (in one sensor group)provides a higher potential barrier that better prevents leakage ofphotogenerated carriers to the wrong carrier-collecting region of thegroup (or to an adjacent sensor group) than would the potential gradientproduced using the prior-technique.

In addition to a blanket barrier layer of the type discussed withreference to FIG. 20, some embodiments of the invention includeadditional p-type barrier regions formed between carrier-collectingsensor regions at the same depth in adjacent sensor groups (i.e.,“laterally” with respect to each such carrier-collecting sensor region).For example, as shown in FIG. 24, additional barrier regions 207(comprising p-type semiconductor material) can be formed in p-semiconductor material between cathode 200 and cathodes (not shown) atthe same depth in adjacent sensor groups, (e.g., cathodes to the leftand to the right of cathode 200). FIG. 24 also shows additional barrierregions 208 (comprising p-type semiconductor material) formed inp-semiconductor material 204 between cathode 202 and cathodes (notshown) at the same depth in adjacent sensor groups (e.g., cathodes tothe left and to the right of cathode 202). Laterally-positionedadditional barrier regions 207 and 208 change the potential gradientbetween the carrier-collecting sensor regions of the adjacent sensorgroups so as to reduce the risk that photogenerated carriers (electronsin the embodiment shown), generated at a location near to a firstcathode (e.g., cathode 200) will drift to a cathode located farther awaythan the first cathode (e.g., to a cathode of another sensor group, notshown in FIG. 24, positioned to the right of cathode 200).

Additional barriers 207 (and 208) are preferably formed using aself-aligned complementary implant process such as that to be describedwith reference to FIGS. 25A-25D. Alternatively, they can be maskedseparately. As shown in FIG. 25A, SiO₂ screen 209 is produced on layer201, a Si₃N₄ mask deposited on the SiO₂ screen, the mask is etched fromthe region at which cathode 200 is to be formed, and an ion implantationprocedure then produces n-type cathode 200 below the exposed portion ofscreen 209. As shown in FIG. 25B, a blocking layer of SiO₂ is then grownon the exposed portion of screen 209. As shown in FIG. 25C, the Si₃N₄mask is then stripped away and another ion implantation procedure isthen performed to produce p-type barriers 207. Finally, as shown in FIG.25D, additional SiO₂ is grown over the exposed SiO₂ surface of theentire structure to minimize the step height between portions of theexposed SiO₂ surface.

Various methods can be used to deposit a semiconductor material on topof other semiconductor materials or insulating materials duringfabrication of a VCF sensor group. One method is the physical transferof material from one wafer to another and the bonding of that materialto the final wafer. This leaves islands of sensor material on thesubstrate. These can be insulated by the dielectric of the passivation,which is yet another version of dielectric isolation. Bonded wafers canbe fabricated with leakage and yield characteristics as good as those ofbulk wafers, especially where the fabrication process produces a thermalSi/SiO2 interface in the bonded wafer.

With reference to FIGS. 14A-14L, we next explain how several of theabove-mentioned fabrication techniques are employed to fabricate one ofthe VCF sensor groups of FIG. 8 in a preferred manner. The preferredfabrication method allows color filters 43 and 48 to be inexpensivelyincluded in an array of VCF sensor groups. The fabrication techniques tobe described with reference to FIGS. 14A-14L (and variations thereon)can be employed to manufacture other embodiments of the inventive VCFsensor group and arrays thereof, as well as to manufacture some types ofsemiconductor integrated circuits (e.g., circuits includingtransistors).

FIG. 14A shows the result of performing the first steps in the processsequence, which are to implant n-type layer 41 in p-type substrate 40,and then grow SiO2 layer 42 on substrate 40 by a thermal oxide growthoperation. Alternatively, layer 42 (and layers 44, 47, and 49) can bemade of another dielectric material, for example, silicon nitride (SiN).

FIG. 14B shows the result of performing the next step in the processsequence, which is to deposit “red pass/cyan reflect” filter 43 on layer42. Filter 43 can be an interference filter made of alternating layersSiN and SiO2. Alternatively, filter 43 can be an interference filtercomprising layers of materials having different refractive indices(other than layers of SiN and SiO2), preferably materials for whichthere is a deposition recipe that can be performed using conventionalCVD equipment. Filter 43 alternatively is a “red pass/cyan absorb”filter that absorbs but does not significantly reflect green and blueradiation.

FIG. 14C shows the next step in the process sequence, which is to bringa second wafer into contact with the wafer of FIG. 14B. Specifically,the second wafer comprises substrate 45 (of p-type silicon) and SiO2layer 44 (grown on substrate 45). Then, as shown in FIG. 14D, layer 44of the second wafer is bonded to filter 43 of the first wafer(preferably by a thermal bonding step) to cause filter 43 to becomesandwiched between SiO2 layers 42 and 44. More generally, bonding of twowafers (each having some layers of the inventive VCF sensor group formedthereon) can be used during manufacture of the invention. Any of avariety of known bonding techniques can be used to manufacture typicalembodiments of the invention, such as those described in Pasquariello,et al., “Plasma-Assisted InP-to-Si Low Temperature Wafer Bonding, IEEEJournal on Selected Topics in Quantum Electronics, Vol. 8, No. 1,January/February 2002.

FIG. 14E shows the result of performing the next step in the processsequence, which is to reduce the thickness of p-type substrate 45 to adesired thickness, if such thickness-reduction is needed. This is doneby polishing the exposed surface of substrate 45 back to a thickness ofabout 0.5 μm, or by cleaving, or by some other means.

FIGS. 14F and 14G show the results of performing the next steps in theprocess sequence, which are to implant n-type layer 46 in substrate 45,then grow SiO2 layer 47 on the exposed (top) surface of substrate 45 bya thermal oxide growth operation (as shown in FIG. 14F), and then todeposit filter layer 48 (which can, but need not, consist of SiNmaterial) on SiO2 layer 47 as shown in FIG. 14G. FIG. 14H shows the nextstep in the process sequence, which is to bring a third wafer intocontact with the bonded and processed wafers of FIG. 14G. Then, as shownin FIG. 141, layer 49 of the third wafer is bonded to the exposed (top)surface of layer 48 (preferably by a thermal bonding step) to causelayer 48 to become sandwiched between SiO2 layers 47 and 49.

Layers 47, 48, and 49 (as shown in FIG. 14I) together comprise aninterference filter that finctions as a “yellow pass/blue reflect”filter. Alternatively, an interference filter comprising more than threealternating layers SiN and SiO2 can be formed by producing a stack ofadditional layers of SiN and SiO2 on the structure of FIG. 14G beforebonding a third wafer (of the type shown in FIG. 14H) to the top of thestack. In other alternative embodiments, an interference filtercomprising a stack of layers of materials having different refractiveindices (a stack that does not consist of layers of SiN and SiO2) can beformed on the FIG. 14E structure, before a third wafer (of the typeshown in FIG. 14H but possibly having a layer of some material otherthan SiO2 in place of SiO2 layer 49 of FIG. 14H) is bonded to the top ofthe stack. Filter 47,48, and 49 is alternatively a “yellow pass/blueabsorb” filter that absorbs but does not significantly reflect blueradiation. The third wafer, shown in FIG. 14H, comprises substrate 50(of p-type silicon) and SiO2 layer 49 (grown on substrate 50). Any of avariety of known bonding techniques can be used to accomplish thebonding step described with reference to FIG. 14I, including at leastsome of those described in the above-cited paper by Pasquariello, et al.

FIG. 14J shows the result of performing the next step in the processsequence, which is to reduce the thickness of p-type substrate 50 to adesired thickness, if such thickness reduction is needed. This is doneby polishing the exposed surface of substrate 50 back to a thickness ofabout 0.3 μm, or by cleaving, or by some other means.

FIG. 14K shows the result of performing the next step in the processsequence, which is to implant n-type layer 51 in substrate 50. Then, asshown in FIG. 14L, final CMOS processing steps are performed. Thesefinal steps can include passivation, formation of contacts (orcompletion of the process of forming contacts, and mounting of lightshield 54 in the appropriate position.

To use the final structure shown in FIG. 14L, a contact extending fromeach of layers 41, 46, and 51 to the exposed (top) surface of thestructure would need to be fabricated. The contacts would preferably beformed in any of the ways described herein or alternatively, asdescribed in application Ser. No. 09/884,863. With reference to FIGS.15-15H, we next describe one preferred technique for fabricating eachsuch contact.

The technique to be described with reference to FIGS. 15A-15H forms alow leakage trench contact, preferably using a trench etcher of the typenormally used for a high performance analog bipolar (or DRAM) process.

FIG. 15A shows the result of performing the first step in the processsequence, which is to etch a trench through silicon layers 50 and 51 ofthe FIG. 14L structure to dielectric layer 49.

Next, as shown in FIG. 15B, an appropriate etch process (e.g., an oxideetch process when layers 47, 48, and 49 consist of SiN or SiO2) extendsthe trench to silicon layer 45. Next, as shown in FIG. 15C, a siliconetch process extends the trench to dielectric layer 44. Next, as shownin FIG. 15D, an appropriate etch process (e.g., an oxide etch processwhen layers 44, 43, and 42 consist of SiN or SiO2) extends the trench tosilicon layer 40.

Next, as shown in FIG. 15E, a timed silicon etch process extends thetrench into n-type silicon cathode layer 41 (the cathode of the redsensor).

Next, as shown in FIG. 15F, the trench is lined with an insulator,preferably by growing SiO₂ passivation layer 301 in all exposed surfacesof the trench. Next, as shown in FIG. 15G, an anisotropic etch isperformed into cathode layer 41 to remove the insulator from the trenchbottom only and expose the n-type silicon material of cathode layer 41.

Finally, as shown in FIG. 15H, the trench is filled with n-typepolysilicon material 302 to complete the trench contact to layer 41. Thetop of the trench contact can be coupled directly to a biasing andreadout circuit (e.g., to the gate of a source-follower amplifiertransistor 56 r of FIG. 2A).

In a block of solid material in which the inventive VCF sensor group isformed, trenches can be produced and filled with semiconductor materialto form contacts to buried sensor cathodes and anodes. For example,semiconductor material around a trench can be doped and a passivationlayer then grown on the doped lining of the trench, the bottom of thetrench can then be opened (e.g., by an anisotropic etch), and the openedtrench can then be filled with an n-type semiconductor (e.g.,n+polysilicon) so that it finctions as an n-type contact to a buriedn-type cathode. Alternatively, such a trench can be lined and/or filledwith insulating material to isolate VCF sensor groups from each other.Trench contacts (or isolating structures) can be made much narrower thancan plug contacts formed by diffusion as described in application Ser.No. 09/884,863. A trench having cross-sectional area of 0.5 μm and adepth of a few microns can easily be produced using existing techniquesto form a trench contact to a deep sensor in a typical VCF sensor group.Such a cross-sectional area is much less than the minimumcross-sectional area of a diffused plug contact (having the same depth)that can be inexpensively produced using existing techniques. Use oftrench contacts (or trench isolating structures) can improve the fillfactor of an array of horizontally separated VCF sensor groups, in thesense that they can increase the area of the imaging plane in whichincident radiation can be detected by sensors of VCF sensor groups (anddecrease the area of the imaging plane that is blocked by radiationshields or occupied by structures that do not convert incident radiationinto detectable electrons or holes).

In preferred embodiments, at least one plug contact is formed in a VCFsensor group by a multi-stage implantation process that produces thediffused plug contact with a cross-sectional area much less than theminimum cross-sectional area of a diffused plug contact (having the samedepth) than can be inexpensively produced using existing techniques. Asshown in FIG. 17, an n-type plug contact to an n-type cathode of a “red”sensor (at a depth of about 2 μm below the n-type cathode of a “green”sensor, and about 2.6 μm below the top surface of the finished sensorgroup) can be formed by the prior technique of implanting phosphorus(with energy 1200 KeV) into an exposed surface of p-type silicon (at adepth of about 1.3 μm from the top surface of the finished sensor group)to form a bottom portion of the contact, then forming additionalstructure (including a p-type silicon epitaxial layer) above the exposedsurface, and then implanting phosphorus (with energy 500 KeV) into thenew exposed surface of p-type silicon (at a depth of about 0.6 μm fromthe top surface of the finished sensor group) to form a top portion ofthe contact. However, as is also shown in FIG. 17, this results in acontact having undesirably large diameter (a diameter of as much as 2.2μm or more, depending on the n-type doping level employed and the numberof thermal cycles of the process). Furthermore, the need to place athick (e.g., 3 μm) photoresist layer on the sensor group duringfabrication of the contact (to prevent the high energy, e.g., 1200 KeV,phosphorus implants from reaching undesired areas of the sensor group)imposes a minimum on the size of the sensor group features that can beformed.

In contrast with the technique employed to produce the FIG. 17 contact,we next describe (with reference to FIGS. 18 and 18A) an embodiment of amulti-stage implantation process performed in accordance with thepresent invention. The inventive multi-stage implantation process ofFIGS. 18 and 18A is performed after the target (e.g., red sensor cathode310 of FIG. 18, consisting of n-type silicon) has been formed (e.g., byimplanting Arsenic with energy 60 KeV into a p-type substrate), and canproduce a contact having diameter of about 0.5 μm that extends to atarget at a depth of about 2 μm in a sensor group. The process includesfour steps.

The first step is to form a first epitaxial layer (epi layer) is on thetarget to which the contact is to extend (e.g., layer 311 of p-typeSilicon is formed on photodiode cathode 310 as shown in FIG. 18).

A bottom portion of the plug (e.g., plug portions 312 and 313 of FIG.18) is then formed by ion implantation in the first epi layer (311). Todo so, a thin nitride mask 314 can be formed on layer 311, a small maskopening 318 (having diameter of about 0.5 μm) then produced in mask 314,and Arsenic then implanted through opening 318, in the typical case thatlayer 311 has thickness of about 1 μm so that the bottom portion of theplug need extend only a short distance (1 μm) through layer 311. Withsuch a mask and such first epi layer thickness, a first part 312 of theplug's bottom portion (extending from layer 310 to about 0.7 μm abovelayer 310) can be formed by implanting Arsenic with energy 1200 Kev intolayer 311, and a second part 313 of the plug's bottom portion (extendingabout 0.3 μm from part 312 to the top surface of layer 311) can then beformed on part 312 by implanting Arsenic with energy 500 KeV into layer311.

An advantage of implanting a substance having diffusivity lower thanthat of Phosphorus (e.g., Arsenic) in accordance with the invention isthat this allows use of a much thinner mask, as is apparent frominspection of FIG. 19. FIG. 19 is a graph of the mask thickness requiredduring typical implantation of Boron, Phosphorus, Arsenic, and Antimony,for each of five indicated masking materials. For example, FIG. 19indicates that a mask of Si₃N₄ having thickness of about 0.07 μm can beused during Arsenic implantation (at 100 KeV) whereas a mask of Si₃N₄having thickness greater than 0.15 μm would be needed during Phosphorusimplantation at the same energy.

The third step is to remove mask 314 from first epi layer 311, and thenform a second epitaxial layer (epi layer 315 of FIG. 18A, consisting ofp-type Silicon) on first epi layer 311.

A top portion of the plug (e.g., plug portions 316 and 317 of FIG. 18A)is then formed by ion implantation in the second epi layer (315). To doso, a thin nitride mask 319 can be formed on layer 315, a small maskopening 320 (having diameter of about 0.5 μm) then produced in mask 319,and Arsenic then implanted through opening 320, in the typical case thatlayer 315 has thickness of about 1 μm so that the top portion of theplug need extend only a short distance (1 μm) through layer 315. Withsuch a mask and such second epi layer thickness, a first part 316 of theplug's bottom portion (extending from layer 311 to about 0.7 μm abovelayer 311) can be formed by implanting Arsenic with energy 1200 Kev intolayer 315, and a second part 317 of the plug's bottom portion (extendingabout 0.3 μm from part 316 to the top surface of layer 315) can then beformed on part 316 by implanting Arsenic with energy 500 KeV into layer315.

More generally, a class of embodiments of the invention use a substancehaving diffusivity lower than that of phosphorus (preferably, Arsenic(“As”) rather than the conventionally-used Phosphorus (“P”)) to performthe implantation steps required for diffused plug formation. Such asubstance (having low diffusivity) diffuses less horizontally than doesPhosphorus, thus allowing narrower plugs to be formed so that sensorgroups can be manufactured with an improved fill factor. AlthoughArsenic has much lower diffusivity (both vertical and horizontaldiffusivity) than Phosphorus, the inventive multi-stage implantationprocess (of which a typical example has been described with reference toFIGS. 18, 18A, and 19) makes it practical to implant Arsenic (ratherthen Phosphorus) to form a diffused plug. This is because the Arseniconly needs to diffuse a relatively short distance vertically througheach epi layer in the multi-stage implantation process; not through along distance (e.g., all the way from the top of the sensor group to theburied target to which the contact is to extend) as in conventionalmethods for diffused plug formation.

Variations on the multi-stage implantation process described above (withreference to FIGS. 18, 18A, and 19) employ a low diffusivity substanceother than Arsenic and/or more than three epi layers above the target. Adifferent portion of the contact is formed in each epi layer.

When manufacturing a VCF sensor group (or array of VCF sensor groups) ona wafer, at least one transistor (for use in coupling at least onesensor of each VCF sensor group) can be formed on the “bottom” surfaceof the wafer (the surface opposite the “top” surface of the group atwhich radiation to be sensed is incident). Formation of such transistorson the bottom surface of the wafer (rather than the top surface of thegroup) improves the fill factor for an array of horizontally separatedVCF sensor groups. Transistors can be formed on the bottom surface of awafer in many different embodiments of the inventive VCF sensor groupand arrays of VCF sensor groups.

An example of a method for forming a VCF sensor group on a wafer with atransistor on the bottom surface of a wafer will be described withreference to FIGS. 16A-16H. FIGS. 16A-16H assume that the structurecomprising elements 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, and 51(shown in FIG. 16A) has been formed in advance. This structure isidentical to that shown in FIG. 14K, and will be referred to as the“main” structure. The description of the main structure and the methodfor manufacturing it will not be repeated.

As shown in FIG. 16A, a “handle” wafer comprising p-type semiconductorsubstrate material 91 and insulating layer 90 on substrate 91 is thenaligned with the main structure, with top layer 50 of the main structurefacing insulating layer 90 of the handle wafer.

Then, as shown in FIG. 16B, layer 90 of the handle wafer is bonded tothe exposed (top) surface of layer 50 (preferably by a thermal bondingstep) to cause layer 90 to become sandwiched between p-typesemiconductor layer 50 and p-type semiconductor substrate 91.

The exposed bottom surface of substrate 40 is then polished back toreduce its thickness (as shown in FIG. 16C) and thus make the sensors(comprising red sensor cathode layer 41, green sensor cathode layer 46,and blue sensor cathode layer 51) accessible from the bottom. Theresulting structure can then be inverted, as shown in FIG. 16C so thatthe exposed “bottom” surface of polished element 40 is at the top ofFIG. 16C.

As shown in FIG. 16D, a trench contact (96) is then formed to extendfrom the exposed “bottom” surface of element 40 (at the top of FIG. 16D)to blue sensor cathode layer 51. This can be done in the mannerdescribed with reference to FIGS. 15A- I 5H. Support circuitry 92 isthen formed on the exposed bottom surface of element 40, preferably by asemiconductor integrated circuit fabrication process. Support circuitry92 includes at least one transistor coupled to the bottom of trenchcontact 96 (at the top of FIG. 16D). Another trench contact (not shown)is formed from the exposed bottom surface of element 40 to green sensorcathode layer 46, and a third trench contact (not shown) is formed fromthe exposed bottom surface of element 40 to red sensor cathode layer 41.At least one transistor of support circuitry 92 is coupled, via a trenchcontact, to each of layers 41, 46, and 51.

As shown in FIG. 16E, a second “handle” wafer comprising p-typesemiconductor substrate material 94 and insulating layer 93 on substrate94 is then aligned with the FIG. 16D structure, with the exposed(bottom) surface of the p-type semiconductor substrate of element 92facing insulating layer 93.

Then, as shown in FIG. 16F, layer 93 of the second handle wafer isbonded to the exposed surface of element 92 (preferably by a lowtemperature bonding step) to cause layer 93 to become sandwiched betweenthe p-type semiconductor substrate of element 92 and p-typesemiconductor substrate 94.

Then substrate 91 is removed (e.g., polished away) and the structure ofFIG. 16F can be inverted (so that the exposed bottom surface ofsubstrate 94 faces down and the exposed top surface of layer 90 faces upas shown in FIG. 16G).

Support circuitry 92 can then be coupled to biasing and readoutcircuitry. For example, as shown in FIG. 16H, support circuitry 92 canbe coupled to biasing and readout circuit 96 by shell-case structure 95,which implements contacts between each transistor of support circuitry92 and circuit 96. Commercially used methods (e.g., those developed byShellcase Ltd.) can be employed to produce shell-case structure 95.Biasing and readout circuit 96 can be of the type described withreference to FIG. 2A.

Another method of creating isolation (e.g., between neighboring VCFsensor groups) is to use a shut off MOS transistor as an isolationstructure. This can be done with a thick oxide transistor having a gatethat surrounds the top layer of the sensor group to be isolated (wherethe gate kept at a voltage well below threshold), or with another typeof MOS transistor. A shut off MOS transistor is useful for isolatingsemiconducting regions near a surface, but does not greatly affect pathsdeep in the substrate. It may therefore be best applied in combinationwith isolation methods of the type described above with reference toFIGS. 20-24, to isolate neighboring VCF sensor groups from each other.

An example of the isolation method mentioned in the previous paragraphis ring isolation, which can be implemented by forming a thick or thinoxide MOS transistor whose gate surrounds the top layer of the sensorgroup to be isolated. In operation, the gate is biased to shut off thetransistor.

Any of a number of available methods can be used to fabricate a VCFsensor group, and best method in each case depends on the materials andrequirements for the sensor group.

Structures in silicon can be constructed with epitaxial growth andimplantation, as described for example in above-referenced U.S. patentapplication Ser. No. 09/884,863. Ion implantation provides a method ofconstructing junction structure below the silicon surface. By using highenergy (>400 KeV) implantation, deep structures are possible. BecauseVCF sensor groups typically require thicker silicon structures than canbe created even with high energy implantation, epitaxial growth incombination with implantation is typically employed to create the deepstructures needed for capturing photons (in accordance with theinvention) by converting the photons to electron/hole pairs deep in thesilicon.

Another method employed to create deep structure in some embodiments ofthe invention is silicon bonding. This method bonds, at a molecularlevel, a layer of one semiconducting or insulating material to another.For example, it is possible to create structure in one silicon wafer andthen bond a thin layer of silicon to the top of it. It is also becomingpossible to bond dissimilar semiconductors. For example, with propermaterial preparation, a III-V semiconductor can be bonded to silicon.Because of the dissimilarity in the expansion coefficient of the twomaterials, an island of III-V material on a volume of silicon cannot belarge. However, an island of III-V material (e.g., the In_(x)Ga_(1-x)Nmaterial discussed above with reference to FIG. 7) that is sufficientlylarge for forming typical embodiments of the inventive VCF sensor groupcan be bonded to silicon. An important advantage of doing so is that theIII-V material can be chosen to absorb radiation in a differentwavelength band than does silicon (e.g., some III-V material transmitsall or substantially all green and red radiation incident thereonalthough silicon has significant absorptivity to green radiation andmuch greater absorptivity to green than to red radiation). Thus a sensorgroup can be implemented in which each sensor formed from III-V materialabsorbs radiation in a different wavelength band than does each sensorformed from silicon underlying the III-V material.

To add a filter to a vertical structure (e.g., a VCF color filter), itis possible to create a trench (or other void) in a volume ofsemiconductor material and then fill the void with a filter materialthat is a liquid or other fluid (e.g., a slurry). One method ofaccomplishing this is to use lateral silicon overgrowth to form a voidin a volume of semiconductor material, and then to etch away the oxide(present during the lateral silicon overgrowth step). A liquid etchant,such as hydrofluoric acid, can be used for the etching step. When a voidhas been formed under silicon, the void can be filled with a liquidoptical filter material (or with optical filter material that is a fluidother than a liquid). This filter material would be solidified (e.g., byheat treatment or UV treatment) to form the VCF sensor group structure.Alternatively, an oxide region could be formed by ion implantation ofoxygen followed by a reaction phase that would create SiO2 (silicondioxide) from the reaction of the wafer and the implanted oxygen.

The process described in the previous paragraph will next be describedin greater detail with reference to FIGS. 13 a-13 f. FIG. 13 a showsSiO2 region 170 formed on the surface of p-type semiconductor 171 (whichcan be silicon), and implanted n-type semiconductor region 172 creatinga p-n junction under SiO2 region 170. Implanted region 172 will becomeone of the sensors of a VCF sensor group. FIG. 13 b also shows a firstplug implant (of n-type semiconductor material) extending upward fromthe right edge of region 172.

FIG. 13 b shows the same cross section after lateral epitaxial growthhas covered SiO2 region 170 with additional p-type semiconductormaterial 171 of the same type as semiconductor 171 of FIG. 13 a (whichcan be silicon). Lateral epitaxial growth has been used in thesemiconductor industry to create dielectrically isolated, single crystalsilicon. As shown in FIG. 13 c, a near surface implant (of n-typesemiconductor material) is then formed over SiO2 region 170, and asecond plug implant (of n-type semiconductor material) is formed toextend upward to the top surface of semiconductor 171 from the firstplug implant. The two plug implants together form a plug contact forcoupling layer 172 to biasing and readout circuitry.

As shown in FIG. 13 d, the next step is to etch away enough of material171 to form a trench that exposes underlying SiO2 region 170. Then, anSiO2 etch is performed to remove the oxide (the SiO2) from region 170,leaving a void below top layer 173 as shown in FIG. 13 e. Finally, thevoid is filled with liquid filter material 174 (as shown in FIG. 13 f)and material 174 is solidified. Alternatively, filter material 174 is afluid other than a liquid.

Variations on the method described with reference to FIGS. 13 a-13 f canbe used to form VCF sensor groups with two or more vertically-separatedsensors below the filter region (the filled with filter material 174).

In some embodiments of the inventive VCF sensor group, semiconductingmaterials other than crystalline silicon are deposited on a wafer orother substrate. Two examples of such semiconducting materials areamorphous silicon and polysilicon.

Amorphous silicon can be deposited by a variety of chemical vapordeposition and sputtering techniques. Amorphous silicon can depositedwith high quality by plasma assisted chemical vapor deposition using SiHas the source gas. Doping of the deposited amorphous silicon can beachieved by adding small amounts of other hydride, such as phosphine,arsine and diborane. Amorphous silicon can be used in a VCF sensor groupas a sensor (by creating a pn diode within the amorphous silicon), or asa filter, or as both a filter and a sensor. Amorphous silicon has beenused in photoimaging arrays. The low temperature (less than 400° C.) atwhich amorphous silicon is deposited is an advantage because it onlyslightly increases diffusion of dopants and may be compatible with somefilters.

In a similar fashion, polysilicon can be formed on a semiconductor waferor other substrate. Typically, amorphous silicon is deposited and thenre-crystallized to form polysilicon. Polysilicon can be doped byimplantation or from a deposited layer to create a pn junction.Transistors can also be formed in either amorphous silicon orpolysilicon and used in addressing sensors of VCF sensor groups.

Various filters and combinations of filters can be included in the VCFsensor groups of the present invention to provide improved photonseparation, color accuracy, and sensor resolution. For example, an arrayof VCF sensor groups can be combined with organic color filters of thetype typically used in image sensor manufacturing. Filters can be formedon (or included in) a subset of the sensor groups of the array in acheckerboard-like pattern to tune the color response of the sensorgroups that are responsive to blue and red illumination. With such apattern of filters, the properties of each filter can be very simple andinsensitive to manufacturing variations due to the fact that the filterworks in conjunction with the semiconductor color filter properties ofeach VCF sensor group. The advantage gained is a potentially moredesirable color filter response. Alternatively, organic, dielectric, orpolysilicon filters can be placed on (or included in) a subset of theVCF sensor groups of an array, in an alternating arrangement such thatevery other sensor group that responds to a particular color also has acolor filter that serves to shape the color response, thereby creatingan array with six distinct color responses. The latter technique allowsfor a large variety of color responses while minimizing themanufacturing overhead associated with the process of placing organicfilters (or other types of filters) on top of an image sensor surface orincluding filters in VCF sensor groups.

While best modes for implementing the present invention and applicationsof the invention have been described herein, it will be apparent tothose of ordinary skill in the art that many variations on theembodiments and applications described herein are possible withoutdeparting from the scope of the invention described and claimed herein.It should be understood that while certain forms of the invention havebeen shown and described, the invention is not to be limited to thespecific embodiments described and shown or the specific methodsdescribed. Further, the claims that describe methods do not imply anyspecific order of steps unless explicitly described in the claimlanguage.

1. (canceled)
 2. (canceled)
 3. (canceled)
 4. (canceled)
 5. (canceled)
 6. An array of sensor groups formed on a semiconductor substrate, said array including: a first sensor group including at least two vertically stacked sensors, wherein the sensors of the first sensor group include a top sensor having a top surface that defines a normal axis, each of the sensors of the first sensor group has a different spectral response, is configured to be biased to function as a photodiode, and has a carrier-collection element configured to collect photo-generated carriers when said sensor is biased to function as a photodiode; and a second sensor group including at least two vertically stacked sensors, wherein each of the sensors of each of the second sensor group has a different spectral response, is configured to be biased to function as a photodiode, and has a carrier-collection element configured to collect photo-generated carriers when said sensor is biased to function as a photodiode, wherein the carrier-collection element of at least one of the sensors of the first sensor group is a relatively large element having an area, projected on a plane perpendicular to the normal axis, that is larger than the area projected on said plane of the carrier-collection element of another one of the sensors of the first sensor group, and the relatively large carrier-collection element is included in and shared by both the first sensor group and the second sensor group.
 7. The array of claim 6, wherein each of the first sensor group and the second sensor group includes: a top, blue-sensitive sensor; a bottom, red-sensitive sensor; and a green-sensitive sensor between the blue-sensitive sensor and the red-sensitive sensor.
 8. The array of claim 7, wherein the relatively large carrier-collection element is a carrier-collection element of a red-sensitive sensor that is shared by the first sensor group and the second sensor group.
 9. The array of claim 7, wherein the relatively large carrier-collection element is a carrier-collection element of a blue-sensitive sensor that is shared by the first sensor group and the second sensor group.
 10. The array of claim 6, wherein each of the sensors of the first sensor group is configured to collect photo-generated carriers of a first polarity in response to incident radiation when said each of the sensors is biased to function as a photodiode, each of the sensors of the second sensor group is configured to collect photo-generated carriers of a first polarity in response to incident radiation when said each of the sensors is biased to function as a photodiode, and said array also includes: circuitry, coupled to the sensors of the first sensor group and the second sensor group and configured to convert the photo-generated carriers collected by the first sensor group into at least one electrical signal and to convert the photo-generated carriers collected by the second sensor group into at least one other electrical signal.
 11. The array of claim 10, wherein each of the sensors of the first sensor group includes at least one reference layer vertically stacked with the carrier-collecting layer of said each of the sensors, and the reference layer is configured to collect and conduct away photo-generated carriers of a polarity opposite to the first polarity.
 12. The array of claim 6, also including: at least one filter positioned relative to the sensors of the first sensor group such that radiation that has propagated through or reflected from the filter will propagate into at least one of the sensors of the first sensor group.
 13. (canceled)
 14. (canceled)
 15. (canceled)
 16. The sensor group of claim 22, wherein the filter has a structure and position relative to the sensors such that filtered radiation that has propagated through said filter is incident on at least one of the sensors.
 17. The sensor group of claim 22, wherein the filter has a structure and position relative to the sensors such that filtered radiation that has reflected from said filter is incident on at least one of the sensors.
 18. The sensor group of claim 22, wherein the filter comprises at least one layer that has been integrated with the sensors by a semiconductor integrated circuit fabrication process.
 19. (canceled)
 20. A sensor group formed on a semiconductor substrate, said sensor group comprising: at least two vertically stacked sensors including a top sensor having a top surface that defines a normal axis, wherein each of the sensors has a different spectral response, is configured to be biased to function as a photodiode, and has a carrier-collection element configured to collect photo-generated carriers when the sensors are biased to function as photodiodes, wherein at least one said carrier-collection element is a minimum-sized carrier-collection element, and the carrier-collection element of at least one of the sensors has an area, projected on a plane perpendicular to the normal axis, that is substantially larger than the area, projected on said plane, of each said minimum-sized carrier-collection element, wherein the sensors include: a top, blue-sensitive sensor; a bottom, red-sensitive sensor; and a green-sensitive sensor between the blue-sensitive sensor and the red-sensitive sensor, the group includes one minimum-sized carrier-collection element, the carrier-collection element of the green-sensitive sensor is the minimum-sized carrier-collection element, and the carrier-collection element of at least one of the red-sensitive sensor and the blue-sensitive sensor has an area, projected on the plane perpendicular to the normal axis, that is at least twice the area, projected on said plane, of the minimum-sized carrier-collection element.
 21. A sensor group formed on a semiconductor substrate, said sensor group comprising: at least two vertically stacked sensors including a top sensor having a top surface that defines a normal axis, wherein each of the sensors has a different spectral response, is configured to be biased to function as a photodiode, and has a carrier-collection element configured to collect photo-generated carriers when the sensors are biased to function as photodiodes, wherein at least one said carrier-collection element is a minimum-sized carrier-collection element, and the carrier-collection element of at least one of the sensors has an area, projected on a plane perpendicular to the normal axis, that is substantially larger than the area, projected on said plane, of each said minimum-sized carrier-collection element, wherein the sensors are configured such that when radiation, propagating along each axis that extends from the top surface through all the sensors, is incident at the group, the radiation is incident at the top sensor with an incidence angle not greater than 30 degrees with respect to the normal axis.
 22. A sensor group formed on a semiconductor substrate, said sensor group comprising: at least two vertically stacked sensors, each having a different spectral response and configured to be biased to function as a photodiode to collect photo-generated carriers of a first polarity in response to incident radiation, wherein the sensors include a top sensor having a top surface that defines a normal axis; circuitry coupled to the sensors and configured to convert the photo-generated carriers into at least one electrical signal; and at least one filter positioned relative to the sensors such that radiation that has propagated through or reflected from the filter will propagate into at least one of the sensors, wherein each of the sensors has a carrier-collection element configured to collect photo-generated carriers of the first polarity when the sensors are biased to function as photodiodes, at least one said carrier-collection element is a minimum-sized carrier-collection element, and the carrier-collection element of at least one of the sensors has an area, projected on a plane perpendicular to the normal axis, that is substantially larger than the area, projected on said plane, of each said minimum-sized carrier-collection element.
 23. A sensor group formed on a semiconductor substrate, said sensor group comprising: at least two vertically stacked sensors, each having a different spectral response and configured to be biased to function as a photodiode to collect photo-generated carriers of a first polarity in response to incident radiation, wherein the sensors include a top sensor having a top surface that defines a normal axis; and circuitry coupled to the sensors and configured to convert the photo-generated carriers into at least one electrical signal, wherein each of the sensors has a carrier-collection element configured to collect photo-generated carriers of the first polarity when the sensors are biased to function as photodiodes, at least one said carrier-collection element is a minimum-sized carrier-collection element, the carrier-collection element of at least one of the sensors has an area, projected on a plane perpendicular to the normal axis, that is substantially larger than the area, projected on said plane, of each said minimum-sized carrier-collection element, and the sensors are configured such that when radiation, propagating along each axis that extends from the top surface through all the sensors, is incident at the group, the radiation is incident at the top sensor with an incidence angle not greater than 30 degrees with respect to the normal axis. 